Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended			12 Months Ended	3 Months Ended				3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013 Revolving credit facility	Dec. 31, 2012 Revolving credit facility	Mar. 31, 2013 Interest rate contracts	Dec. 31, 2012 Interest rate contracts	Mar. 31, 2013 Interest rate contracts Cash Flow Hedging	Dec. 31, 2012 Interest rate contracts Cash Flow Hedging	Mar. 31, 2013 Interest rate contracts Revolving credit facility item	Dec. 31, 2012 Interest rate contracts Revolving credit facility item
Derivative instruments
Variable rate of debt	one-month LIBOR	one-month LIBOR	one-month LIBOR	LIBOR
Borrowings at variable interest rates					$ 347.0	$ 282.0
Number of interest rate swaps held									1	1
Notional amount outstanding							100.0		100.0	100.0
Remaining maturity term									8 months
Fixed interest rate (as a percent)									2.10%
Net fair value of swap liability							1.2	1.7
Estimated increase in net finance costs					$ 1.3