Arbitrage Trading Account (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Trading Securities and Derivative Instruments [Abstract]
Arbitrage trading account	$ 450,648,000	$ 522,128,000
Options Held | Long
Derivative [Line Items]
Other derivatives not designated as hedging instruments at fair value, net	8,000,000
Derivative, notional amount	133,000,000
Options Held | Short
Derivative [Line Items]
Other derivatives not designated as hedging instruments at fair value, net	2,000,000
Derivative, notional amount	$ 106,000,000