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Arbitrage Trading Account and Arbitrage Funds (Fair Value and Carrying Value of the Arbitrage Trading Account and Arbitrage Funds and Related Assets and Liabilities) (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2013
Dec. 31, 2012
Arbitrage Trading Account And Arbitrage Funds [Abstract]    
Arbitrage trading account $ 701,223 $ 329,077