Derivative Instruments - Interest rate swap agreement (Details) - Interest rate contract	12 Months Ended
	Dec. 31, 2018 USD ($) item	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Derivative instruments
Borrowings at variable interest rates	$ 427,000,000	$ 501,300,000
Number of interest rate swap agreements | item	1
Notional amount outstanding	$ 100,000,000	100,000,000
Remaining maturity term	28 months
Gain (loss) recorded to net finance costs	$ (400,000)	600,000	$ 25,000
Net fair value of swap liability	$ 1,700,000	$ 1,100,000