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Derivative Instruments - Interest rate swap agreement (Details) - Interest rate contract $ in Thousands | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2019 USD ($) agreement | Sep. 30, 2018 USD ($) | Sep. 30, 2019 USD ($) agreement | Sep. 30, 2018 USD ($) | Oct. 03, 2019 USD ($) | Dec. 31, 2018 USD ($)
Derivative [Line Items] |  |  |  |  |  |
Borrowings at variable interest rates | $ 384,000 |  | $ 384,000 |  |  | $ 427,000
Number of interest rate swap agreements | agreement | 1 |  | 1 |  |  |
Derivative, notional amount | $ 100,000 |  | $ 100,000 |  |  | 100,000
Remaining maturity term |  |  | 19 months |  |  |
Net fair value of swap liability | 245 |  | $ 245 |  |  |
Net fair value of swap asset |  |  |  |  |  | $ 1,700
Gain (loss) recorded to net finance costs | $ 100 | $ 100 | $ 500 | $ 200 |  |
Subsequent Event |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Derivative, notional amount |  |  |  |  | $ 100,000 |

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