Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Derivative instruments
Variable rate of debt	one-month LIBOR	one-month LIBOR
Revolving credit facility
Derivative instruments
Variable rate of debt	LIBOR
Interest rate contracts
Derivative instruments
Borrowings at variable interest rates	$ 339.0	$ 282.0
Estimated increase in net finance costs	0.8
Interest rate contracts | Cash Flow Hedging
Derivative instruments
Notional amount outstanding	100.0
Net fair value of swap liability	0.8	1.7
Interest rate contracts | Revolving credit facility
Derivative instruments
Number of interest rate swaps held	1	1
Notional amount outstanding	$ 100.0	$ 100.0
Remaining maturity term	5 months
Fixed interest rate (as a percent)	2.10%	2.10%