SENIOR NOTES PAYABLE AND OTHER DEBT - Summary of Senior Notes Payables and Other Debt (Details) $ in Thousands, CAD in Millions	Jun. 30, 2017 USD ($)	Jun. 30, 2017 GBP (£)	Jun. 30, 2017 CAD	Apr. 30, 2017	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2016 GBP (£)	Dec. 31, 2016 CAD
Debt instruments
Principal amount outstanding	$ 11,992,233					$ 11,190,914
Deferred financing costs, net	(73,679)					(61,304)
Unamortized fair value adjustment	20,696					25,224
Unamortized discounts	(31,253)					(27,508)
Senior notes payable and other debt	11,907,997					11,127,326
Unsecured revolving credit facilities
Debt instruments
Principal amount outstanding	516,455	£ 30,400,000	CAD 33.1			146,538	£ 0	CAD 146.5
1.250% Senior Notes due 2017
Debt instruments
Principal amount outstanding	$ 0					300,000
Interest rate	1.25%	1.25%	1.25%	1.25%
2.00% Senior Notes due 2018
Debt instruments
Principal amount outstanding	$ 700,000					700,000
Interest rate	2.00%	2.00%	2.00%
Unsecured Term Loan Due 2018
Debt instruments
Principal amount outstanding	$ 200,000					200,000
Unsecured term loan due 2019
Debt instruments
Principal amount outstanding	278,586					371,215		CAD 92.6
4.00% Senior Notes due 2019
Debt instruments
Principal amount outstanding	$ 600,000					600,000
Interest rate	4.00%	4.00%	4.00%
3.00% Senior Notes, Series A due 2019
Debt instruments
Principal amount outstanding	$ 308,595					297,841
Interest rate	3.00%	3.00%	3.00%
2.700% Senior Notes due 2020
Debt instruments
Principal amount outstanding	$ 500,000					500,000
Interest rate	2.70%	2.70%	2.70%
Unsecured term loan due 2020
Debt instruments
Principal amount outstanding	$ 900,000					900,000
4.750% Senior Notes due 2021
Debt instruments
Principal amount outstanding	$ 700,000					700,000
Interest rate	4.75%	4.75%	4.75%
4.25% Senior Notes due 2022
Debt instruments
Principal amount outstanding	$ 600,000					600,000
Interest rate	4.25%	4.25%	4.25%
3.25% Senior Notes due 2022
Debt instruments
Principal amount outstanding	$ 500,000					500,000
Interest rate	3.25%	3.25%	3.25%
3.300% Senior Notes due 2022
Debt instruments
Principal amount outstanding	$ 192,871					186,150
Interest rate	3.30%	3.30%	3.30%
3.125% Senior Notes due 2023
Debt instruments
Principal amount outstanding	$ 400,000					400,000
Interest rate	3.125%	3.125%	3.125%
3.100% Senior Notes due 2023
Debt instruments
Principal amount outstanding	$ 400,000				$ 400,000	0
Interest rate	3.10%	3.10%	3.10%		3.10%
2.55% Senior Notes, Series D due 2023
Debt instruments
Principal amount outstanding	$ 212,159					0
3.750% Senior Notes due 2024
Debt instruments
Principal amount outstanding	$ 400,000					400,000
Interest rate	3.75%	3.75%	3.75%
4.125% Senior Notes, Series B due 2024
Debt instruments
Principal amount outstanding	$ 192,872					186,150
Interest rate	4.125%	4.125%	4.125%
3.500% Senior Notes due 2025
Debt instruments
Principal amount outstanding	$ 600,000					600,000
Interest rate	3.50%	3.50%	3.50%
4.125% Senior Notes due 2026
Debt instruments
Principal amount outstanding	$ 500,000					500,000
Interest rate	4.125%	4.125%	4.125%
3.25% Senior Notes due 2026
Debt instruments
Principal amount outstanding	$ 450,000					450,000
Interest rate	3.25%	3.25%	3.25%
3.850% Senior Notes due 2027
Debt instruments
Principal amount outstanding	$ 400,000				$ 400,000	0
Interest rate	3.85%	3.85%	3.85%		3.85%
6.90% Senior Notes due 2037
Debt instruments
Principal amount outstanding						52,400
Interest rate	6.90%	6.90%	6.90%
6.59% Senior Notes due 2038
Debt instruments
Principal amount outstanding						22,973
Interest rate	6.59%	6.59%	6.59%
5.45% Senior Notes due 2043
Debt instruments
Principal amount outstanding	$ 258,750					258,750
Interest rate	5.45%	5.45%	5.45%
5.70% Senior Notes due 2043
Debt instruments
Principal amount outstanding	$ 300,000					300,000
Interest rate	5.70%	5.70%	5.70%
4.375% Senior Notes due 2045
Debt instruments
Principal amount outstanding	$ 300,000					300,000
Interest rate	4.375%	4.375%	4.375%
Mortgage loans and other
Debt instruments
Principal amount outstanding	$ 1,506,572					1,718,897
Unsecured term loan
Debt instruments
Principal amount outstanding	$ 478,600					$ 571,200