Debt - Debt Summary (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Unsecured debt	$ 3,113,216	$ 2,617,482
Lines of credit	0	246,391
Secured Debt	2,322,837	2,246,255
Unsecured Debt And Line Of Credit	5,436,053	5,110,128
Fixed Rate Bond One [Member]
Debt Instrument [Line Items]
Unsecured debt	465,000	465,000
Debt Instruments Maturity Period	4 years
Loans Payable [Member]
Debt Instrument [Line Items]
Unsecured debt	225,000	225,000
Debt Instruments Maturity Period	1 year 7 months 6 days
Long-term Debt, Weighted Average Interest Rate	2.40%	2.40%
Fixed Rate Bond Two [Member]
Debt Instrument [Line Items]
Unsecured debt	2,423,216	1,927,482
Debt Instruments Maturity Period	7 years 6 months
Long-term Debt, Weighted Average Interest Rate	3.60%	3.60%
Unsecured Line of Credit [Member]
Debt Instrument [Line Items]
Lines of credit	0	246,391
Long-term Debt, Weighted Average Interest Rate	0.00%	1.80%
Mortgage Notes [Member]
Debt Instrument [Line Items]
Debt Instruments Maturity Period	6 years 7 months
Secured Debt	$ 2,322,837	$ 2,246,255
Long-term Debt, Weighted Average Interest Rate	4.50%	4.60%