Derivative Instruments - Interest rate swap agreement (Details) - Interest rate contract $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) item	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) item	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Derivative instruments
Borrowings at variable interest rates	$ 466.0		$ 466.0		$ 501.3
Number of interest rate swap agreements | item	1		1
Notional amount outstanding	$ 100.0		$ 100.0		100.0
Remaining maturity term			31 months
Gain (loss) recorded to net finance costs	0.1	$ (0.1)	$ 0.2	$ (0.5)
Net fair value of swap liability	$ 2.8		$ 2.8		$ 1.1