Derivative Instruments - Notional and Fair Value Amounts of Derivative Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Notional Amount	$ 246,800	$ 186,100
Fair Value	15,550	8,694
Bank of America, N.A.
Derivative [Line Items]
Notional Amount	85,100	38,500
Fair Value	6,320	1,934
Barclays Bank PLC
Derivative [Line Items]
Notional Amount	48,900	66,800
Fair Value	1,828	1,543
Citigroup Inc.
Derivative [Line Items]
Notional Amount	0	0
Fair Value	0	0
Credit Suisse International
Derivative [Line Items]
Notional Amount	21,100	65,200
Fair Value	1,444	4,281
Societe Generale
Derivative [Line Items]
Notional Amount	91,700	15,600
Fair Value	$ 5,958	$ 936