Accounting rulemakers have published another new accounting rule proposal, this one targeting the transparency of multi-employer pension and other post-retirement benefit plans.

The rule would require companies that participate in such plans, such as union pension plans and retirement benefit plans, to provide more information about the fiscal health of the plans they are obligated to support. The Financial Accounting Standards Board, which is proposing the update to the Accounting Standards Codification, says the top 100 multi-employer plans in the United States were underfunded to the tune of $160 billion in 2008, yet current accounting rules make no requirements for companies paying into the plans to explain their obligations related to that mounting deficit to their investors.

Instead companies currently are only required to disclose their historical contributions, FASB said in a summary of the proposal. Companies have told FASB it was difficult in the past to get information from the pension plan sponsors to pass along to investors, but the Pension Protection Act of 2006 increased employers’ access to information about such plans. FASB said companies now can use that data to provide more disclosure to their investors.

FASB’s proposal would require companies to describe the plans they’re obligated to fund on behalf of their employees, the company’s contractual commitment to such funds, and the expected impact on the company’s future cash flow. “We heard from a number of investors and other users of financial statements that existing disclosures do not provide enough information about an employer’s involvement in multi-employer plans, making it hard to evaluate the potential impact on the company,” said board member Larry Smith in a FASB podcast explaining the proposal. The proposal focuses only on disclosure, he said, making no changes to the accounting requirements.

Companies would be required to disclose, for example, the total assets and accumulated benefit obligation of the plan, data depicting the scope of the employer’s participation in the plan, discussion of the contractual arrangement, expected contributions for the next annual period, and any known trends in future contributions. Companies also would be required to give a number to the amount they would have to pay to withdraw from the plan.

FASB is accepting comments on the proposal through Nov. 1, with a target to put the disclosures in place for the 2010 financial statements for calendar-year public companies.