Derivative Instruments - Interest rate swap agreement (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) item	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) item	Jun. 30, 2017 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Derivative instruments
Gain (loss) recorded to net finance costs	$ 94	$ (200)
Net fair value of swap liability	2,500		$ 2,500
Interest rate contracts
Derivative instruments
Borrowings at variable interest rates	$ 666,000		$ 666,000			$ 501,300
Number of interest rate swap agreements | item	1		1
Notional amount outstanding	$ 100,000		$ 100,000		$ 100,000
Remaining maturity term			34 months
Gain (loss) recorded to net finance costs			$ (69,000)	$ (400)
Net fair value of swap liability						$ 1,100