Long-Term Debt -Interest Rate Caps (Details) (USD $) In Millions, unless otherwise specified						12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Cap agreement effective January 14, 2013-2015 [Member] derivatives	Mar. 31, 2013 Minimum [Member]	Mar. 31, 2013 Interest Rate Cap Purchased During 2011 [Member]	Dec. 31, 2012 Interest Rate Cap [Member] derivatives	Dec. 31, 2011 Interest Rate Cap [Member] derivatives	Mar. 31, 2013 Interest Rate Cap [Member] Interest Rate Cap Purchased During 2012 [Member]
Derivative [Line Items]
Interest Rate Cap, hedge percent requirement				50.00%
Interest Rate Cap, number of agreements			10
Interest Rate Cap, notional amount			$ 1,150.0
Interest Rate Cap, cap rate					3.50%			1.50%
Number of Interest Rate Derivatives Purchased						6	4
Interest Rate Cap, fair value	$ 0.1	$ 0.1