Derivative Instruments - Gross and Net Derivative Positions, Including Pledged Collateral (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Offsetting Derivative Assets
Gross amounts recognized	$ 3,659	$ 717
Gross amounts offset in the Consolidated Balance Sheets	0	0
Net amounts presented in the Consolidated Balance Sheets	3,659	717
Financial instruments	884	717
Financial collateral pledged	0	0
Net amounts	2,775	0
Gross amounts not offset in the Consolidated Balance Sheets
Gross amounts recognized	3,111	5,303
Gross amounts offset in the Consolidated Balance Sheets	0	0
Net amounts presented in the Consolidated Balance Sheets	3,111	5,303
Financial instruments	884	717
Financial collateral pledged	175	4,357
Net amounts	$ 2,052	$ 229