The accounting is set by ASC 715. A company must recognize each defined benefit plan's funded status on its balance sheet. An underfunded plan is a liability, and an overfunded plan is an asset. For pensions, the obligation is measured as the projected benefit obligation, which includes expected future salary increases. The portion of an unfunded plan's benefits expected to be paid over the next year from company assets, rather than from the plan, is classified as current. In XBRL, the noncurrent liability is usually tagged PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent, or DefinedBenefitPensionPlanLiabilitiesNoncurrent for pensions alone, and the plan-level funded status is disclosed as DefinedBenefitPlanFundedStatusOfPlan.
The obligation rests on actuarial assumptions, and the discount rate matters most. When interest rates rise, the present value of future benefits falls and the liability shrinks, and when rates fall it grows, even if nothing about the plan changes. Changes from updated assumptions and investment returns generally pass through other comprehensive income first rather than hitting earnings immediately. The pension footnote lists the assumptions and shows how the balance moved.
Because a pension deficit is a claim on future cash, much like debt, many analysts add the after-tax unfunded amount to net debt or enterprise value for companies with large legacy plans, typically industrials, airlines, automakers, and utilities. Required cash contributions and the plan's asset mix, disclosed in the same note, show how quickly the gap may need to be closed.