Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2014 item	Mar. 31, 2013	Dec. 31, 2013 item
Derivative instruments
Variable rate of debt	one-month LIBOR
Interest rate contracts
Derivative instruments
Variable rate of debt	one-month LIBOR		one-month LIBOR
Borrowings at variable interest rates	$ 392.3		$ 392.0
Number of interest rate swap agreements	0		0
Notional amount outstanding			100.0
Fixed interest rate (as a percent)			2.10%
Benefit recorded to net finance costs	$ 0.1	$ 0.1