Borrowing Arrangements (Unsecured Revolving Credit Facility and Unsecured Term Loans) (Details) (USD $)	12 Months Ended					1 Months Ended				1 Months Ended				1 Months Ended		1 Months Ended	12 Months Ended											1 Months Ended		1 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 Unsecured credit facility	Dec. 31, 2012 Unsecured credit facility	Dec. 31, 2013 Unsecured term loan due 2015		Dec. 31, 2012 Unsecured term loan due 2015		Dec. 31, 2013 Unsecured term loan due 2017		Dec. 31, 2012 Unsecured term loan due 2017		Dec. 31, 2013 Unsecured term loan due 2018	Dec. 31, 2012 Unsecured term loan due 2018	Dec. 31, 2013 Unsecured revolving credit facilities	Dec. 31, 2013 Unsecured revolving credit facilities	Dec. 31, 2011 Unsecured revolving credit facilities	Dec. 31, 2012 Unsecured revolving credit facilities	Dec. 31, 2013 Unsecured Term Loan Due 2018		Dec. 31, 2012 Unsecured Term Loan Due 2018		Dec. 31, 2013 Unsecured Term Loan Due 2019		Dec. 31, 2012 Unsecured Term Loan Due 2019		Dec. 31, 2013 Unsecured term loan	Dec. 31, 2012 Unsecured term loan	Dec. 31, 2013 London Interbank Offered Rate (LIBOR) Unsecured revolving credit facilities	Dec. 31, 2013 London Interbank Offered Rate (LIBOR) Unsecured Term Loan Due 2018	Dec. 31, 2013 London Interbank Offered Rate (LIBOR) Unsecured Term Loan Due 2019
Debt Instrument [Line Items]
Current borrowing capacity				$ 3,000,000,000	$ 2,000,000,000											$ 2,000,000,000	$ 2,000,000,000			$ 200,000,000				$ 800,000,000
Variable interest rate																														LIBOR	LIBOR	LIBOR
Basis spread on variable rate (as a percent)																														1.00%	1.05%	1.05%
Line of credit facility optional extension period (in years)																1 year
Line of Credit Facility, Maximum Borrowing Capacity				3,500,000,000
Debt amount, prepaid in full						125,000,000				375,000,000				180,000,000														680,000,000
Principal amount outstanding	9,320,477,000	8,186,315,000				0	[1]	130,336,000	[1]	0	[1]	375,000,000	[1]	0	180,000,000	376,343,000	376,343,000		540,727,000	200,000,000	[2]	0	[2]	800,702,000	[2]	0	[2]
Amount outstanding																												1,000,000,000	500,000,000
Letters of credit outstanding																14,900,000	14,900,000
Remaining borrowing capacity																1,600,000,000	1,600,000,000
Loss on extinguishment of debt	$ (1,201,000)	$ (37,640,000)	$ (27,604,000)														$ 1,500,000	$ 2,400,000

[1] These amounts represent in aggregate the approximate $500.0 million of borrowings outstanding under our previous unsecured term loan facility. Certain amounts included in the 2015 tranche were in the form of Canadian dollar borrowings.

[2] These amounts represent in aggregate the approximate $1.0 billion of unsecured term loan borrowings under our new unsecured credit facility. Certain amounts included in the 2019 tranche are in the form of Canadian dollar borrowings.