Indebtedness Secured Debt (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
gross basis of secured properties	$ 1,660,000,000
Letters of Credit Outstanding, Amount	3,500,000
Fair value	4,778,176,000	4,503,707,000
Repayments of Secured Debt	112,877,000	169,188,000	117,287,000
Fixed Rate Secured Debt [Member]
Debt Instrument [Line Items]
Fair value	1,065,301,000	1,145,717,000
Debt Instrument, Interest Rate, Effective Percentage Rate Range, Minimum	2.20%
Debt Instrument, Interest Rate, Effective Percentage Rate Range, Maximum	3.60%
Number Of Secured Loans Assumed In Property Acquisition		3
Debt Instrument, Face Amount		99,300,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt		103,600,000
Weighted Average Interest Rate Secured Loans Assumed In Property Acquisition		5.59%
Debt Instrument, Interest Rate, Effective Percentage		3.00%
number of secured loans repaid	9	12
Repayments of Secured Debt	$ 99,300,000	$ 153,800,000
Repaid Debt, Weighted Average Interest Rate	5.56%	5.52%
Assumed Debt - Weighted Average Remaining Term		1 year 9 months 9 days