Arbitrage Trading Account (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Trading Securities and Derivative Instruments [Abstract]
Arbitrage trading account	$ 452,548	$ 617,649
Options Held | Long
Derivative [Line Items]
Other derivatives not designated as hedging instruments at fair value, net	37
Derivative, notional amount	18,400
Options Held | Short
Derivative [Line Items]
Other derivatives not designated as hedging instruments at fair value, net	58
Derivative, notional amount	$ 11,600