Derivative Instruments - Interest rate swap agreement (Details) - Interest rate contract $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2019 USD ($) agreement	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) agreement	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Derivative [Line Items]
Borrowings at variable interest rates	$ 397,000		$ 397,000		$ 427,000
Number of interest rate swap agreements \| agreement	1		1
Notional amount outstanding	$ 100,000		$ 100,000		100,000
Notional amount, liability	(22)		$ (22)
Remaining maturity term			22 months
Net fair value of swap liability	22		$ 22
Net fair value of swap asset					$ 1,700
Gain (loss) recorded to net finance costs	$ 200	$ 100	$ 400	$ 100

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Period remaining until the derivative contract matures, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Number of interest rate derivative instruments held by the entity at the reporting date.

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Fair value portion of liabilities pertaining to principal and customer trading transactions, or which may be incurred with the objective of generating a profit from short-term fluctuations in price as part of an entity's market-making, hedging and proprietary trading. Examples include, but are not limited to, short positions in securities, derivatives and commodities.

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