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Arbitrage Trading Account (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Trading Securities and Derivative Instruments [Abstract]    
Arbitrage trading account $ 617,649 $ 299,999
Options Held | Long    
Derivative [Line Items]    
Other derivatives not designated as hedging instruments at fair value, net 1,000  
Derivative, notional amount 136,000  
Options Held | Short    
Derivative [Line Items]    
Other derivatives not designated as hedging instruments at fair value, net 8,000  
Derivative, notional amount $ 135,000