Indebtedness - Credit Facility (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Line of Credit Facility [Line Items]
Line of Credit Facility, Amount Outstanding	$ 0	$ 92,000,000
Total Assets for Covenant Ratio	6,775,560,000	5,174,179,000
Proceeds from unsecured notes	448,646,000	700,000,000	0
Line of Credit [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Expiration Date	Nov. 01, 2015
Length of Maturity Extension	1
Competitive Bid Option % of Facility Amount	50.00%
Line of Credit Facility, Amount Outstanding	0
Line of Credit Facility, Commitment Fee Percentage	0.20%
Debt Instrument, Basis Spread on Variable Rate	1.075%
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	200.00%