Derivative Instruments - Gross and Net Derivative Positions, Including Pledged Collateral (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Offsetting Derivative Assets
Gross amounts recognized	$ 6,168	$ 717
Gross amounts offset in the Consolidated Balance Sheets	0	0
Net amounts presented in the Consolidated Balance Sheets	6,168	717
Financial instruments	553	717
Financial collateral pledged	0	0
Net amounts	5,615	0
Offsetting Derivative Liabilities
Gross amounts recognized	1,334	5,303
Gross amounts offset in the Consolidated Balance Sheets	0	0
Net amounts presented in the Consolidated Balance Sheets	1,334	5,303
Financial instruments	553	717
Financial collateral pledged	781	4,357
Net amounts	$ 0	$ 229

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