Derivative Instruments (Details) - Interest rate contracts $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) item	Dec. 31, 2014 USD ($) item
Derivative instruments
Variable rate of debt	one-month LIBOR	one-month LIBOR
Borrowings at variable interest rates	$ 562.1	$ 468.5
Number of interest rate swap agreements | item	0	1
Notional amount outstanding		$ 100.0
Fixed interest rate (as a percent)		2.10%
(Benefit) expense recorded to net finance costs	$ (0.5)	$ 1.5