Derivative Instruments - Location and Amounts of Interest Rate Swap Gains and Losses (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Net unrealized gain (loss) on financial instruments	$ 2,157	$ (150)	$ 18,371	$ (849)
Derivatives Not Designated as Hedging Instruments | Net unrealized gain on interest rate swap agreements | Interest rate swaps
Derivative Instruments, Gain (Loss) [Line Items]
Net unrealized gain (loss) on financial instruments	2,340	(150)	19,792	(849)
Derivatives Not Designated as Hedging Instruments | Financing interest income | Interest rate swaps
Derivative Instruments, Gain (Loss) [Line Items]
Net unrealized gain (loss) on financial instruments	$ 1,866	$ 377	$ 3,542	$ (243)