Borrowing Arrangements (Summary of Senior Notes Payable and Other Debt) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Principal amount outstanding	$ 11,271,020	$ 10,888,295
Deferred financing costs, net	0	0
Unamortized fair value adjustment	33,570	42,516
Unamortized discounts	(28,473)	(26,132)
Senior notes payable and other debt	11,206,996	10,844,351
Unsecured revolving credit facilities
Debt Instrument [Line Items]
Principal amount outstanding	180,683	919,099
Unsecured revolving credit facilities | Canadian dollars
Debt Instrument [Line Items]
Principal amount outstanding	$ 9,700	$ 164,100
3.125% Senior Notes due 2015
Debt Instrument [Line Items]
Interest rate (as a percent)	3.125%	3.125%
Principal amount outstanding	$ 0	$ 400,000
6% Senior Notes due 2015
Debt Instrument [Line Items]
Interest rate (as a percent)	6.00%	6.00%
Principal amount outstanding	$ 0	$ 234,420
1.55% Senior Notes due 2016
Debt Instrument [Line Items]
Interest rate (as a percent)	1.55%	1.55%
Principal amount outstanding	$ 550,000	$ 550,000
1.250% Senior Notes due 2017
Debt Instrument [Line Items]
Interest rate (as a percent)	1.25%	1.25%
Principal amount outstanding	$ 300,000	$ 300,000
2.00% Senior Notes due 2018
Debt Instrument [Line Items]
Interest rate (as a percent)	2.00%	2.00%
Principal amount outstanding	$ 700,000	$ 700,000
Unsecured term loan due 2018
Debt Instrument [Line Items]
Principal amount outstanding	200,000	200,000
Unsecured term loan due 2019
Debt Instrument [Line Items]
Principal amount outstanding	$ 468,477	$ 790,634
4.00% Senior Notes due 2019
Debt Instrument [Line Items]
Interest rate (as a percent)	4.00%	4.00%
Principal amount outstanding	$ 600,000	$ 600,000
3.00% Senior Notes, Series A due 2019
Debt Instrument [Line Items]
Interest rate (as a percent)	3.00%	3.00%
Principal amount outstanding	$ 289,038	$ 344,204
2.700% Senior Notes due 2020
Debt Instrument [Line Items]
Interest rate (as a percent)	2.70%	2.70%
Principal amount outstanding	$ 500,000	$ 500,000
Unsecured term loan due 2020
Debt Instrument [Line Items]
Principal amount outstanding	$ 900,000	$ 0
4.750% Senior Notes due 2021
Debt Instrument [Line Items]
Interest rate (as a percent)	4.75%	4.75%
Principal amount outstanding	$ 700,000	$ 700,000
4.25% Senior Notes due 2022
Debt Instrument [Line Items]
Interest rate (as a percent)	4.25%	4.25%
Principal amount outstanding	$ 600,000	$ 600,000
3.25% Senior Notes due 2022
Debt Instrument [Line Items]
Interest rate (as a percent)	3.25%	3.25%
Principal amount outstanding	$ 500,000	$ 500,000
3.300% Senior Notes due 2022
Debt Instrument [Line Items]
Interest rate (as a percent)	3.30%	3.30%
Principal amount outstanding	$ 180,649	$ 0
3.750% Senior Notes due 2024
Debt Instrument [Line Items]
Interest rate (as a percent)	3.75%	3.75%
Principal amount outstanding	$ 400,000	$ 400,000
4.125% Senior Notes, Series B due 2024
Debt Instrument [Line Items]
Interest rate (as a percent)	4.125%	4.125%
Principal amount outstanding	$ 180,649	$ 215,128
3.500% Senior Notes due 2025
Debt Instrument [Line Items]
Interest rate (as a percent)	3.50%
Principal amount outstanding	$ 600,000	0
4.125% Senior Notes due 2026
Debt Instrument [Line Items]
Interest rate (as a percent)	4.125%
Principal amount outstanding	$ 500,000	$ 0
6.90% Senior Notes due 2037
Debt Instrument [Line Items]
Interest rate (as a percent)	6.90%	6.90%
Principal amount outstanding	$ 52,400	$ 52,400
6.59% Senior Notes due 2038
Debt Instrument [Line Items]
Interest rate (as a percent)	6.59%	6.59%
Principal amount outstanding	$ 22,973	$ 22,973
5.45% Senior Notes due 2043
Debt Instrument [Line Items]
Interest rate (as a percent)	5.45%	5.45%
Principal amount outstanding	$ 258,750	$ 258,750
5.70% Senior Notes due 2043
Debt Instrument [Line Items]
Interest rate (as a percent)	5.70%	5.70%
Principal amount outstanding	$ 300,000	$ 300,000
4.375% Senior Notes due 2045
Debt Instrument [Line Items]
Interest rate (as a percent)	4.375%
Principal amount outstanding	$ 300,000	0
Mortgage loans and other
Debt Instrument [Line Items]
Principal amount outstanding	1,987,401	2,300,687
Senior Notes and Other Debt
Debt Instrument [Line Items]
Deferred financing costs, net	(69,121)	(60,328)
Unsecured term loan
Debt Instrument [Line Items]
Principal amount outstanding	668,000
Unsecured term loan | Canadian dollars
Debt Instrument [Line Items]
Principal amount outstanding	89,900
Mortgage loans and others
Debt Instrument [Line Items]
Principal amount outstanding	$ 22,900	$ 27,600