Derivative Instruments (Details)	12 Months Ended
	Dec. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Derivative instruments
Number of interest rate swap agreements | item	1
Notional amount outstanding	$ 100,000,000
Remaining maturity term	52 months
(Benefit) expense recorded to net finance costs	$ 25,000	$ 0	$ 500,000
Net fair value of swap liability	68,000
Interest rate contracts
Derivative instruments
Borrowings at variable interest rates	$ 619,700,000	$ 562,100,000