Derivative Instruments - Notional and Fair Value Amounts of Derivative Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Notional Amount	$ 255,900	$ 246,800
Fair Value	12,630	15,550
Bank of America, N.A.
Derivative [Line Items]
Notional Amount	93,800	85,100
Fair Value	6,017	6,320
Barclays Bank PLC
Derivative [Line Items]
Notional Amount	48,600	48,900
Fair Value	902	1,828
Citigroup Inc.
Derivative [Line Items]
Notional Amount	0	0
Fair Value	0	0
Credit Suisse International
Derivative [Line Items]
Notional Amount	21,100	21,100
Fair Value	1,469	1,444
Societe Generale
Derivative [Line Items]
Notional Amount	92,400	91,700
Fair Value	$ 4,242	$ 5,958