Derivative Instruments - Gross and Net Derivative Positions, Including Pledged Collateral (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Offsetting Derivative Assets
Gross amounts recognized	$ 3,232	$ 717
Gross amounts offset in the Consolidated Balance Sheets	0	0
Net amounts presented in the Consolidated Balance Sheets	3,232	717
Financial instruments	2,913	717
Financial collateral pledged	0	0
Net amounts	319	0
Gross amounts not offset in the Consolidated Balance Sheets
Gross amounts recognized	3,449	5,303
Gross amounts offset in the Consolidated Balance Sheets	0	0
Net amounts presented in the Consolidated Balance Sheets	3,449	5,303
Financial instruments	2,913	717
Financial collateral pledged	231	4,357
Net amounts	$ 305	$ 229

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