Derivative Instruments - Interest rate swap agreement (Details) - Interest rate contracts $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) item	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Derivative instruments
Borrowings at variable interest rates	$ 604.0		$ 501.3
Number of interest rate swap agreements | item	1
Notional amount outstanding	$ 100.0
Remaining maturity term	37 months
Expense recorded to net finance costs	$ (24.0)	$ (0.2)
Net fair value of swap liability	$ 2.2		$ 1.1