Derivative Instruments and Hedging Activities (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended					3 Months Ended	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012	Dec. 31, 2012 Designated as Hedging Instrument [Member] Interest Rate Cap [Member]	Dec. 31, 2011 Designated as Hedging Instrument [Member] Interest Rate Cap [Member]	Sep. 30, 2012 Designated as Hedging Instrument [Member] Swap [Member]	Dec. 31, 2011 Designated as Hedging Instrument [Member] Swap [Member]	Dec. 31, 2010 Designated as Hedging Instrument [Member] Swap [Member]	Dec. 31, 2012 Designated as Hedging Instrument [Member] Interest Rate Swap [Member]
Derivative [Line Items]
Notional amount of interest rate contracts				$ 187.8					$ 300.0
Amount of unsecured loan that is effectively fixed with the interest rate swap									300
Total amount of unsecured loan									350
Interest rate (in hundredths)									2.70%
Number of derivative instruments held				12
Tax exempt variable rate debt			201.9
Aggregate carrying value of the interest rate swap contracts									6.6
Aggregate carrying value of the interest rate cap contracts				0	0.2
Derivative swap transaction terminated						38.0
Derivative instruments settled							20.0	355.0
Proceeds from derivatives contracts settlement							2.3	81.3
Mortgage obtained during 2011							32.0
Effective interest rate on mortgage loans, maximum (in hundredths)	6.20%	6.80%
Effective interest rate on mortgage loans, minimum (in hundredths)	5.40%	4.50%
Term of mortgage loan paid from derivatives proceeds		10 years
Expense related to the ineffectiveness included in impairment and other charges		$ 2.3