The Federal Reserve, which has long enjoyed a tight cloak of secrecy around its financial decision making, lost a key ruling in federal court on Monday that will require it to turn over documents detailing which banks received emergency loans during the height of the financial crisis in 2008. The case stemmed from a lawsuit filed by Bloomberg News after its Freedom of Information Act requests were denied by the national bank, which insisted the public disclosure of loan records would harm borrowers’ competitive positions and endanger the economy.