U.S. companies have given politically active nonprofits at least $185 million in a single year, according to a seven-month investigation by the Center for Public Integrity (CPI).

The center released the information on Wednesday after digging through voluntary disclosures filed by the 300 largest companies in the U.S., as ranked by Fortune. Most of the disclosures covered calendar year 2012.

Who were the biggest donors? Power plant operator Exelon Corp. (NYSE: EXC)($26.2 million), health insurer WellPoint Inc. (NYSE: WLP)($19.2 million) and software and electronics creator Microsoft Corp. (Nasdaq: MSFT)($12.5 million).

But only one-third of the 300 companies voluntarily disclosed payments to politically active nonprofits. Some of the nation’s largest companies, like Wal-Mart Stores Inc. (NYSE:WMT), ExxonMobil Corp. (NYSE:XOM) and AT&T Inc. (NYSE:T) do not disclose their political spending. Nonprofits under sections 501(c)(4) and 501(c)(6) in the U.S. tax code are not required to disclose their sources of funding, unlike candidates, political parties and super PAC’s.

Trade associations received about 84 percent of the $185 million, including America’s Health Insurance Plans, the American Petroleum Institute and the U.S. Chamber of Commerce. The Chamber opposes the DISCLOSE Act, Democratic legislation that would require politically active nonprofits to reveal additional information about their funders.

cpi chart Nonprofits that received funding from top U.S. companies. Photo: Center for Public Integrity

About 13 percent of the $185 million flowed to social welfare nonprofits like the Democratic Third Way think tank and the Republican Main Street Partnership. The remaining 3 percent went to other organizations, like the Congressional Black Caucus Foundation, the National Conference of State Legislators, the Heritage Foundation and the American Legislative Exchange Council.

The $185 million is a minimum estimate, not only because it’s only comprised of self-reported funding, but also because many companies reported only the portion of their payments that used specifically for lobbying and political activities.

Thirty-seven companies reported payments of $1 million or more, including Coca-Cola Co. (NYSE: KO), Aetna Inc. (NYSE: AET) , Intel Corp. (Nasdaq: INTC), PepsiCo Inc. (NYSE: PEP), New York Life Insurance Co., Edison International (NYSE: EIX), Union Pacific Corp. (NYSE: UNP), MetLife Inc. (NYSE: MET), Procter & Gamble Co. (NYSE: PG), General Electric Co. (NYSE: GE), Wells Fargo & Co. (NYSE: WFC) and Chevron Corp. (NYSE: CVX).