Arbitrage Trading Account (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Trading Securities and Derivative Instruments [Abstract]
Arbitrage trading account	$ 376,697	$ 450,648
Options Held | Long
Derivative [Line Items]
Other derivatives not designated as hedging instruments at fair value, net	3,000
Derivative, notional amount	52,000
Options Held | Short
Derivative [Line Items]
Other derivatives not designated as hedging instruments at fair value, net	1,000
Derivative, notional amount	$ 62,000