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Derivative Instruments and Hedging Activities Credit Risk - Counterparty Exposure (Details) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Offsetting Assets and Liabilities [Line Items]    
Derivative assets, at fair value $ 238,305 $ 324,182
Derivative liabilities, at fair value 11,312 $ 12,501
Collateral cash deposits received from counterparties (2,789)  
Collateral cash deposits placed in accounts maintained by counterparties $ 167,629