Debt Notes (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Aug. 23, 2006
Debt Instrument [Line Items]
Interest Rate Derivatives, at Fair Value, Net		$ 300,000,000
Debt Instrument, Interest Rate, Stated Percentage				3.85%
Debt Instrument, Maturity Date	Sep. 01, 2014
Repayments of Notes Payable	400,000,000	975,991,000	575,641,000
Term Loan Facility Spread	1.20%
Summary of Company's unsecured note balances and certain interest rate and maturity date information
Notes, net	5,477,088,000	4,630,875,000
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum	0.03%	0.11%
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Maximum	7.25%	11.25%
Fixed Rate Public or Private Notes [Member]
Summary of Company's unsecured note balances and certain interest rate and maturity date information
Notes, net	4,727,088,000	4,329,352,000
Weighted Average Interest Rate	5.55%	5.70%
Floating Rate Public or Private Notes [Member]
Summary of Company's unsecured note balances and certain interest rate and maturity date information
Notes, net	750,000,000	301,523,000
Weighted Average Interest Rate	1.58%	1.83%
Five Point Two Zero Percentage Fixed Rate Tax Exempt Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount		400,000,000
Debt Instrument, Interest Rate, Stated Percentage		5.20%
Debt Instrument, Maturity Date		Apr. 01, 2013
5.200% Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.20%
Repayments of Notes Payable	400,000,000
DelayedDrawTermLoan [Member]
Debt Instrument [Line Items]
Term Loan Principal Amount	$ 750,000,000
Floating Rate Public or Private Notes [Member]
Summary of Company's unsecured note balances and certain interest rate and maturity date information
Debt Instrument, Maturity Date Range, Start	Jan. 01, 2015	Jan. 01, 2013
Debt Instrument, Maturity Date Range, End	Dec. 31, 2015	Dec. 31, 2013
Fixed Rate Public or Private Notes [Member]
Summary of Company's unsecured note balances and certain interest rate and maturity date information
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum	3.00%	4.625%
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Maximum	7.57%	7.57%
Debt Instrument, Maturity Date Range, Start	Jan. 01, 2014	Jan. 01, 2013
Debt Instrument, Maturity Date Range, End	Dec. 31, 2026	Dec. 31, 2026