Derivative Instruments - Interest rate swap agreement (Details) - Interest rate contracts	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($)
Derivative instruments
Borrowings at variable interest rates	$ 501,300,000	$ 619,700,000
Number of interest rate swap agreements | item		1
Notional amount outstanding	$ 100,000,000	$ 100,000,000
Remaining maturity term	40 months
(Benefit) expense recorded to net finance costs	$ 600,000	25,000	$ 0
Net fair value of swap liability	$ 1,100,000	$ 68,000