Derivative Instruments - Gross and Net Derivative Positions, Including Pledged Collateral (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Offsetting Derivative Assets
Gross amounts recognized	$ 892	$ 4,778
Gross amounts offset in the Consolidated Balance Sheets	0	0
Net amounts presented in the Consolidated Balance Sheets	892	4,778
Financial instruments	780	567
Financial collateral pledged	0	0
Net amounts	112	4,211
Gross amounts not offset in the Consolidated Balance Sheets
Gross amounts recognized	7,879	4,893
Gross amounts offset in the Consolidated Balance Sheets	0	0
Net amounts presented in the Consolidated Balance Sheets	7,879	4,893
Financial instruments	780	567
Financial collateral pledged	6,922	4,326
Net amounts	$ 177	$ 0

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