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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)

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