Indebtedness - Mortgage Loans, Unsecured Notes (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012
Mortgages [Member]
Debt Instrument [Line Items]
Debt Instrument, Maturity Date Range, Start	Jan. 01, 2013
Debt Instrument, Maturity Date Range, End	Dec. 31, 2033
Net Book Value of Collateralized Real Estate Properties	$ 517.9
Fixed Rate Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Percentage Bearing Fixed Interest, Amount	$ 2,549.4
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum	3.00%
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Maximum	7.50%
Mortgage Loans and Unsecured Notes [Member]
Debt Instrument [Line Items]
Weighted Average Remaining Life	6.0