Arbitrage Trading Account (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Trading Securities and Derivative Instruments [Abstract]
Arbitrage trading account	$ 299,999	$ 376,697
Options Held | Long
Derivative [Line Items]
Other derivatives not designated as hedging instruments at fair value, net	1,000
Derivative, notional amount	27,000
Options Held | Short
Derivative [Line Items]
Other derivatives not designated as hedging instruments at fair value, net	2,000
Derivative, notional amount	$ 36,000