SENIOR NOTES PAYABLE AND OTHER DEBT - Summary of Senior Notes Payables and Other Debt (Details)	Sep. 30, 2017 USD ($)	Apr. 30, 2017	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2016 GBP (£)
Debt instruments
Principal amount outstanding	$ 11,511,618,000			$ 11,190,914,000
Deferred financing costs, net	(76,372,000)			(61,304,000)
Unamortized fair value adjustment	18,866,000			25,224,000
Unamortized discounts	(29,967,000)			(27,508,000)
Senior notes payable and other debt	11,424,145,000			11,127,326,000
Unsecured revolving credit facilities
Debt instruments
Principal amount outstanding	538,911,000			146,538,000	£ 0
1.250% Senior Notes due 2017
Debt instruments
Principal amount outstanding	$ 0			300,000,000
Interest rate	1.25%	1.25%
2.00% Senior Notes due 2018
Debt instruments
Principal amount outstanding	$ 700,000,000			700,000,000
Interest rate	2.00%
Unsecured Term Loan Due 2018
Debt instruments
Principal amount outstanding	$ 0			200,000,000
Unsecured term loan due 2019
Debt instruments
Principal amount outstanding	0			371,215,000
4.00% Senior Notes due 2019
Debt instruments
Principal amount outstanding	$ 600,000,000			600,000,000
Interest rate	4.00%
3.00% Senior Notes, Series A due 2019
Debt instruments
Principal amount outstanding	$ 320,847,000			297,841,000
Interest rate	3.00%
2.700% Senior Notes due 2020
Debt instruments
Principal amount outstanding	$ 500,000,000			500,000,000
Interest rate	2.70%
Unsecured term loan due 2020
Debt instruments
Principal amount outstanding	$ 900,000,000			900,000,000
4.750% Senior Notes due 2021
Debt instruments
Principal amount outstanding	$ 700,000,000			700,000,000
Interest rate	4.75%
4.25% Senior Notes due 2022
Debt instruments
Principal amount outstanding	$ 600,000,000			600,000,000
Interest rate	4.25%
3.25% Senior Notes due 2022
Debt instruments
Principal amount outstanding	$ 500,000,000			500,000,000
Interest rate	3.25%
3.300% Senior Notes due 2022
Debt instruments
Principal amount outstanding	$ 200,529,000			186,150,000
Interest rate	3.30%
Secured revolving construction credit facility
Debt instruments
Line of credit facility, fair value of amount outstanding	$ 0
Principal amount outstanding	0			0
3.125% Senior Notes due 2023
Debt instruments
Principal amount outstanding	$ 400,000,000			400,000,000
Interest rate	3.125%
3.100% Senior Notes due 2023
Debt instruments
Principal amount outstanding	$ 400,000,000		$ 400,000,000	0
Interest rate	3.10%		3.10%
2.55% Senior Notes, Series D due 2023
Debt instruments
Principal amount outstanding	$ 220,582,000			0
Interest rate	2.55%
3.750% Senior Notes due 2024
Debt instruments
Principal amount outstanding	$ 400,000,000			400,000,000
Interest rate	3.75%
4.125% Senior Notes, Series B due 2024
Debt instruments
Principal amount outstanding	$ 200,529,000			186,150,000
Interest rate	4.125%
3.500% Senior Notes due 2025
Debt instruments
Principal amount outstanding	$ 600,000,000			600,000,000
Interest rate	3.50%
4.125% Senior Notes due 2026
Debt instruments
Principal amount outstanding	$ 500,000,000			500,000,000
Interest rate	4.125%
3.25% Senior Notes due 2026
Debt instruments
Principal amount outstanding	$ 450,000,000			450,000,000
Interest rate	3.25%
3.850% Senior Notes due 2027
Debt instruments
Principal amount outstanding	$ 400,000,000		$ 400,000,000	0
Interest rate	3.85%		3.85%
6.90% Senior Notes due 2037
Debt instruments
Principal amount outstanding	$ 52,400,000			52,400,000
Interest rate	6.90%
6.59% Senior Notes due 2038
Debt instruments
Principal amount outstanding	$ 22,973,000			22,973,000
Interest rate	6.59%
5.45% Senior Notes due 2043
Debt instruments
Principal amount outstanding	$ 258,750,000			258,750,000
Interest rate	5.45%
5.70% Senior Notes due 2043
Debt instruments
Principal amount outstanding	$ 300,000,000			300,000,000
Interest rate	5.70%
4.375% Senior Notes due 2045
Debt instruments
Principal amount outstanding	$ 300,000,000			300,000,000
Interest rate	4.375%
Mortgage loans and other
Debt instruments
Principal amount outstanding	$ 1,446,097,000			1,718,897,000
Unsecured term loan
Debt instruments
Principal amount outstanding				571,200,000
Borrowings originally denominated in GBP | Unsecured revolving credit facilities
Debt instruments
Principal amount outstanding	31,100,000
Borrowings originally denominated in CAD | Unsecured revolving credit facilities
Debt instruments
Principal amount outstanding	$ 40,800,000			146,500,000
Borrowings originally denominated in CAD | Unsecured term loan due 2019
Debt instruments
Principal amount outstanding				$ 92,600,000