Derivatives (Details 4) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011
Short-term Cap One [Member] | Short term Cap 2011 [Member]
Open interest rate contracts
Year Entered	2011
Year of Maturity	2012
Initial Notional Amount	$ 80.0
Hedged Item	Interest rate component of debt – not accounted for as a hedge
Long-term Swap One [Member] | Long term Swap 2005 [Member]
Open interest rate contracts
Year Entered	2005
Year of Maturity	2016
Initial Notional Amount	4.0
Hedged Item	Interest rate component of an operating lease – not accounted for as a hedge
Interest Rate	5.23%
Long-term Swap Two [Member] | Long term Swap 2006 [Member]
Open interest rate contracts
Year Entered	2006
Year of Maturity	2016
Initial Notional Amount	14.0
Hedged Item	Interest rate component of debt – accounted for as cash flow hedge
Interest Rate	5.95%
Long-term Cap One [Member] | Long term Cap 2011 [Member]
Open interest rate contracts
Year Entered	2011
Year of Maturity	2014
Initial Notional Amount	20.0
Hedged Item	Interest rate component of debt – not accounted for as a hedge
Interest Rate	1.36%
Long-term Cap Two [Member] | Long term Cap 2011 [Member]
Open interest rate contracts
Year Entered	2011
Year of Maturity	2013
Initial Notional Amount	$ 40.0
Hedged Item	Interest rate component of debt – not accounted for as a hedge
Minimum [Member] | Short-term Cap One [Member] | Short term Cap 2011 [Member]
Open interest rate contracts
Interest Rate	0.60%
Minimum [Member] | Long-term Cap Two [Member] | Long term Cap 2011 [Member]
Open interest rate contracts
Interest Rate	1.62%
Maximum [Member] | Short-term Cap One [Member] | Short term Cap 2011 [Member]
Open interest rate contracts
Interest Rate	3.42%
Maximum [Member] | Long-term Cap Two [Member] | Long term Cap 2011 [Member]
Open interest rate contracts
Interest Rate	1.65%