SENIOR NOTES PAYABLE AND OTHER DEBT - Summary of Senior Notes Payables and Other Debt (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Feb. 28, 2018	Dec. 31, 2017
Debt instruments
Senior notes payable and other debt, gross	$ 11,143,235		$ 11,365,633
Deferred financing costs, net	(75,036)		(73,093)
Unamortized fair value adjustment	3,402		12,139
Unamortized discounts	(31,789)		(28,617)
Senior notes payable and other debt	11,039,812		11,276,062
Unsecured revolving credit facilities
Debt instruments
Senior notes payable and other debt, gross	794,368		535,832
Unsecured revolving credit facilities | Borrowings originally denominated in CAD
Debt instruments
Senior notes payable and other debt, gross	35,700		28,700
Unsecured revolving credit facilities | Borrowings originally denominated in GBP
Debt instruments
Senior notes payable and other debt, gross	31,700		31,100
Secured revolving construction credit facility due 2022
Debt instruments
Senior notes payable and other debt, gross	$ 18,632		2,868
2.00% Senior Notes due 2018
Debt instruments
Interest rate	2.00%	2.00%
Senior notes payable and other debt, gross	$ 0		700,000
4.00% Senior Notes due 2019
Debt instruments
Interest rate	4.00%
Senior notes payable and other debt, gross	$ 97,891		600,000
3.00% Senior Notes, Series A due 2019
Debt instruments
Interest rate	3.00%
Senior notes payable and other debt, gross	$ 310,294		318,041
2.700% Senior Notes due 2020
Debt instruments
Interest rate	2.70%
Senior notes payable and other debt, gross	$ 500,000		500,000
Unsecured term loan due 2020
Debt instruments
Senior notes payable and other debt, gross	$ 900,000		900,000
4.750% Senior Notes due 2021
Debt instruments
Interest rate	4.75%
Senior notes payable and other debt, gross	$ 700,000		700,000
4.25% Senior Notes due 2022
Debt instruments
Interest rate	4.25%
Senior notes payable and other debt, gross	$ 600,000		600,000
3.25% Senior Notes due 2022
Debt instruments
Interest rate	3.25%
Senior notes payable and other debt, gross	$ 500,000		500,000
3.300% Senior Notes, Series C due 2022
Debt instruments
Interest rate	3.30%
Senior notes payable and other debt, gross	$ 193,934		198,776
3.125% Senior Notes due 2023
Debt instruments
Interest rate	3.125%
Senior notes payable and other debt, gross	$ 400,000		400,000
3.100% Senior Notes due 2023
Debt instruments
Interest rate	3.10%
Senior notes payable and other debt, gross	$ 400,000		400,000
2.55% Senior Notes, Series D due 2023
Debt instruments
Interest rate	2.55%
Senior notes payable and other debt, gross	$ 213,327		218,653
3.75% Senior Notes due 2024
Debt instruments
Interest rate	3.75%
Senior notes payable and other debt, gross	$ 400,000		400,000
4.125% Senior Notes, Series B due 2024
Debt instruments
Interest rate	4.125%
Senior notes payable and other debt, gross	$ 193,934		198,776
3.500% Senior Notes due 2025
Debt instruments
Interest rate	3.50%
Senior notes payable and other debt, gross	$ 600,000		600,000
4.125% Senior Notes due 2026
Debt instruments
Interest rate	4.125%
Senior notes payable and other debt, gross	$ 500,000		500,000
3.25% Senior Notes due 2026
Debt instruments
Interest rate	3.25%
Senior notes payable and other debt, gross	$ 450,000		450,000
3.850% Senior Notes Due 2027
Debt instruments
Interest rate	3.85%
Senior notes payable and other debt, gross	$ 400,000		400,000
4.00% Senior Notes due 2028
Debt instruments
Interest rate	4.00%	4.00%
Senior notes payable and other debt, gross	$ 650,000		0
6.90% Senior Notes due 2037
Debt instruments
Interest rate	6.90%
Senior notes payable and other debt, gross	$ 52,400		52,400
6.59% Senior Notes due 2038
Debt instruments
Interest rate	6.59%
Senior notes payable and other debt, gross	$ 22,973		22,973
5.45% Senior Notes due 2043
Debt instruments
Interest rate	5.45%
Senior notes payable and other debt, gross	$ 258,750		258,750
5.70% Senior Notes due 2043
Debt instruments
Interest rate	5.70%
Senior notes payable and other debt, gross	$ 300,000		300,000
4.375% Senior Notes due 2045
Debt instruments
Interest rate	4.375%
Senior notes payable and other debt, gross	$ 300,000		300,000
Mortgages loans and other
Debt instruments
Senior notes payable and other debt, gross	$ 1,386,732		$ 1,308,564