Humana posted a $401 million fourth-quarter net loss, primarily because of a previously announced $583 million write-off related to the Affordable Care Act. The Louisville-based insurer also said that it would discuss its 2017 outlook and provide an update on its proposed acquisition by rival Aetna no later than Feb. 16.

A judge had blocked that transaction late last month on antitrust grounds. Aetna said it was considering an appeal and would make a decision by 11:59 p.m. Feb. 15, which is the merger deadline. If the merger falls apart, Aetna, based in Hartford, Conn., may have to pay Humana a $1 billion breakup fee.

Humana said Wednesday that fourth-quarter revenue fell 3.6 percent, to $12.9 billion, while operating expenses, at $13.3 billion, rose 1.9 percent.

The net loss, of $401 million, compared unfavorably to a $101 million profit in the fourth quarter of 2015. Humana’s shares were down about 1.5 percent in early morning trading.

However, the insurer said its loss was primarily attributable to it not being able to collect on $595 million in ACA-related funds it is owed by the federal government.

To keep health insurance premiums stable and to protect insurers against uncertainty in claims’ costs during the first three years of the ACA, also known as Obamacare, the Centers for Medicare and Medicaid Services implemented mechanisms that aimed to spread the insurers’ risk from consumers with lots of health problems and high health care costs.

One of those mechanisms, called the temporary risk corridor, allowed the government to collect money from insurers if the health insurance premiums they collected exceeded claims’ costs by a certain amount and to redistribute those dollars to insurers whose premium collections fell short of claims’ costs by a certain amount.

The problem: The amount of excess profits is being dwarfed by the amount of excess costs. Many big insurers, including Humana and Aetna, have said that the cost of medical care for customers they have gained through the exchanges, which are a central part of the ACA, have exceeded, by far, the health insurance premiums those customers have paid.

CMS said last year that not enough insurers collected excessive profits from ACA customers — and that too many incurred significant losses. That meant the government collected too little risk corridor money from successful insurers to cover the dollars it owed the insurers who struggled with customers they gained through the exchanges.

For payments for the year 2014, CMS collected $362 million in risk corridor payments from successful insurers — but it owed $2.87 billion to the struggling insurers, including Humana.

Bruce Broussard

Without that write-off, Humana said it would have generated pretax income of $643 million, up 42 percent compared to a year earlier.

For the year, the insurer posted net income of $614 million, down 52 percent from 2015. Adjusted for the ACA-related write-off and other items, Humana said pretax income for 2016 would have been $2.8 billion, up 15 percent from 2015.

“We are proud of our strong 2016 operating results, particularly given the complex operating environment during the extended transaction process,” Humana President and CEO Bruce D. Broussard said in a press release. “These results and the current strength of the company are the result of deep clinical engagement with our members, tireless work to simplify the member experience, our partnership with providers and the dedication of our more than 50,000 associates.”