Indebtedness - Credit Facility (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Line of Credit Facility [Line Items]
Number of maturity extensions	2
Long-term Debt, Maturities Total	$ 3,163,395,000
Line of Credit Facility, Amount Outstanding	167,000,000	0
Total Assets for Covenant Ratio	6,625,536,000	6,775,560,000
Line of Credit [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	1.05%
Line of Credit Facility, Expiration Date	Mar. 01, 2018
Length of Maturity Extension	6 months
Line of Credit Facility, Competitive Bid Option, Percentage	50.00%
Long-term Debt, Maturities Total	167,000,000
Line of Credit Facility, Commitment Fee Percentage	0.20%
Line of Credit [Member] | Total Debt to Total Assets Ratio [Member]
Line of Credit Facility [Line Items]
Borrowed Funds for Covenant Ratio	0.60
Total Assets for Covenant Ratio	1
Line of Credit [Member] | EBITDA to Fixed Charges Ratio [Member]
Line of Credit Facility [Line Items]
Earnings Before Interest, Taxes, Depreciation and Amortization for Covenant Ratio	$ 1.50
Line of Credit [Member] | Unsecured Debt to Unencumbered Asset Ratio [Member]
Line of Credit Facility [Line Items]
Unsecured Debt to Unencumbered Asset Value for Debt Covenant	60.00%
Line of Credit [Member] | Unencumbered Net Operating Income to Unsecured Interest Expense [Member]
Line of Credit Facility [Line Items]
Unencumbered Net Operating Income to Unsecured Interest Expense Value for Debt Covenant	175.00%