Derivative Instruments (Details) (Interest rate contracts, USD $) In Millions, unless otherwise specified	12 Months Ended		0 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Sep. 17, 2012 Cash Flow Hedging item	Nov. 18, 2009 Cash Flow Hedging item	Dec. 31, 2012 Cash Flow Hedging	Dec. 31, 2011 Cash Flow Hedging
Derivative instruments
Number of interest rate swaps terminated			6	3
Notional value of interest rate swaps terminated			$ 215.0	$ 105.0
Effective portion of loss on hedge reclassified out of accumulated other comprehensive income and recorded in earnings			(10.2)	2.6
Borrowings at variable interest rates	282.0
Notional amount outstanding	100.0				100.0
Remaining minimum maturity term	3 months	11 years
Remaining maximum maturity term	40 months
Minimum fixed interest rate of derivative instrument (as a percent)	2.10%
Maximum fixed interest rate of derivative instrument (as a percent)	5.05%
Net fair value of swap liability					1.7	12.3
Estimated increase in net finance costs	$ 1.7