Summary of Significant Accounting Policies (Details 2) (Interest rate contracts, USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Derivative instruments
Borrowings at variable rates	$ 339.0		$ 282.0
Revolving credit facility
Derivative instruments
Notional amount of outstanding derivative instruments	100.0		100.0
Cash Flow Hedging
Derivative instruments
Notional amount of outstanding derivative instruments	100.0
Net fair value of swap liability	0.8		1.7
Interest expense	$ 0.8	$ 4.2