Borrowing Arrangements (Summary of Senior Notes Payable and Other Debt) (Details) - USD ($)	Dec. 31, 2016	Jun. 30, 2016	Feb. 29, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Interest rate			1.132%
Principal amount outstanding	$ 11,190,914,000			$ 11,271,020,000
Unamortized fair value adjustment	25,224,000			33,570,000
Unamortized discounts	(27,508,000)			(28,473,000)
Senior notes payable and other debt	11,127,326,000			11,206,996,000
Unsecured revolving credit facilities
Debt Instrument [Line Items]
Principal amount outstanding	146,538,000			180,683,000
Unsecured revolving credit facilities | Canadian dollars
Debt Instrument [Line Items]
Principal amount outstanding	$ 146,500,000			9,700,000
1.55% Senior Notes due 2016
Debt Instrument [Line Items]
Interest rate	1.55%	1.55%
Principal amount outstanding	$ 0			550,000,000
1.250% Senior Notes due 2017
Debt Instrument [Line Items]
Interest rate	1.25%
Principal amount outstanding	$ 300,000,000			300,000,000
2.00% Senior Notes due 2018
Debt Instrument [Line Items]
Interest rate	2.00%
Principal amount outstanding	$ 700,000,000			700,000,000
Unsecured term loan due 2018
Debt Instrument [Line Items]
Principal amount outstanding	200,000,000			200,000,000
Unsecured term loan due 2019
Debt Instrument [Line Items]
Principal amount outstanding	$ 371,215,000			468,477,000
4.00% Senior Notes due 2019
Debt Instrument [Line Items]
Interest rate	4.00%
Principal amount outstanding	$ 600,000,000			600,000,000
3.00% Senior Notes, Series A due 2019
Debt Instrument [Line Items]
Interest rate	3.00%
Principal amount outstanding	$ 297,841,000			289,038,000
2.700% Senior Notes due 2020
Debt Instrument [Line Items]
Interest rate	2.70%
Principal amount outstanding	$ 500,000,000			500,000,000
Unsecured term loan due 2020
Debt Instrument [Line Items]
Principal amount outstanding	$ 900,000,000			900,000,000
4.750% Senior Notes due 2021
Debt Instrument [Line Items]
Interest rate	4.75%
Principal amount outstanding	$ 700,000,000			700,000,000
4.25% Senior Notes due 2022
Debt Instrument [Line Items]
Interest rate	4.25%
Principal amount outstanding	$ 600,000,000			600,000,000
3.25% Senior Notes due 2022
Debt Instrument [Line Items]
Interest rate	3.25%
Principal amount outstanding	$ 500,000,000			500,000,000
3.300% Senior Notes due 2022
Debt Instrument [Line Items]
Interest rate	3.30%
Principal amount outstanding	$ 186,150,000			180,649,000
3.125% Senior Notes due 2023
Debt Instrument [Line Items]
Interest rate	3.125%
Principal amount outstanding	$ 400,000,000			0
3.750% Senior Notes due 2024
Debt Instrument [Line Items]
Interest rate	3.75%
Principal amount outstanding	$ 400,000,000			400,000,000
4.125% Senior Notes, Series B due 2024
Debt Instrument [Line Items]
Interest rate	4.125%
Principal amount outstanding	$ 186,150,000			180,649,000
3.500% Senior Notes due 2025
Debt Instrument [Line Items]
Interest rate	3.50%
Principal amount outstanding	$ 600,000,000			600,000,000
4.125% Senior Notes due 2026
Debt Instrument [Line Items]
Interest rate	4.125%
Principal amount outstanding	$ 500,000,000			500,000,000
3.25% Senior Notes due 2026
Debt Instrument [Line Items]
Interest rate	3.25%
Principal amount outstanding	$ 450,000,000			0
6.90% Senior Notes due 2037
Debt Instrument [Line Items]
Interest rate	6.90%
Principal amount outstanding	$ 52,400,000			52,400,000
6.59% Senior Notes due 2038
Debt Instrument [Line Items]
Interest rate	6.59%
Principal amount outstanding	$ 22,973,000			22,973,000
5.45% Senior Notes due 2043
Debt Instrument [Line Items]
Interest rate	5.45%
Principal amount outstanding	$ 258,750,000			258,750,000
5.70% Senior Notes due 2043
Debt Instrument [Line Items]
Interest rate	5.70%
Principal amount outstanding	$ 300,000,000			300,000,000
4.375% Senior Notes due 2045
Debt Instrument [Line Items]
Interest rate	4.375%
Principal amount outstanding	$ 300,000,000			300,000,000
Mortgage loans and other
Debt Instrument [Line Items]
Principal amount outstanding	1,718,897,000			1,987,401,000
Senior Notes and Other Debt
Debt Instrument [Line Items]
Deferred financing costs, net	(61,304,000)			(69,121,000)
Unsecured term loan
Debt Instrument [Line Items]
Principal amount outstanding	571,200,000
Unsecured term loan | Canadian dollars
Debt Instrument [Line Items]
Principal amount outstanding	92,600,000
Mortgage loans and others
Debt Instrument [Line Items]
Principal amount outstanding	$ 0			$ 22,900,000