SENIOR NOTES PAYABLE AND OTHER DEBT - Narrative (Details) CAD in Thousands	1 Months Ended		3 Months Ended		6 Months Ended
	Jun. 30, 2016 USD ($)	May 31, 2016 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Jul. 27, 2016 USD ($)	Jun. 30, 2016 CAD	May 31, 2016 CAD	Feb. 29, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2015 CAD
Debt instruments
Principal amount outstanding	$ 10,968,271,000		$ 10,968,271,000		$ 10,968,271,000						$ 11,271,020,000
Gain (loss) on extinguishment of debt, net			(2,468,000)	$ 455,000	(2,782,000)	$ 434,000
Derivative notional amount										$ 200,000,000
Derivative fixed interest rate										1.132%
Unsecured revolving credit facilities
Debt instruments
Line of credit facility, current borrowing capacity	2,000,000,000.0		2,000,000,000.0		$ 2,000,000,000.0
Debt instrument, additional period					1 year
Amount outstanding	212,400,000		212,400,000		$ 212,400,000
Letters of credit outstanding	14,100,000		14,100,000		14,100,000
Unused borrowing capacity available	1,700,000,000		1,700,000,000		1,700,000,000
Principal amount outstanding	212,382,000		212,382,000		$ 212,382,000						180,683,000
Unsecured revolving credit facilities | LIBOR
Debt instruments
Debt instrument, basis spread on variable rate					1.00%
Unsecured Term Loan Due 2018
Debt instruments
Line of credit facility, current borrowing capacity	200,000,000.0		200,000,000.0		$ 200,000,000.0
Debt instrument, term of loan					4 years
Principal amount outstanding	200,000,000		200,000,000		$ 200,000,000						200,000,000
Unsecured Term Loan Due 2018 | LIBOR
Debt instruments
Debt instrument, basis spread on variable rate					1.05%
Unsecured term loan due 2019
Debt instruments
Line of credit facility, current borrowing capacity	800,000,000.0		800,000,000.0		$ 800,000,000.0
Debt instrument, term of loan					5 years
Amount outstanding	374,849,000		374,849,000		$ 374,849,000
Repayments of debt					100,000,000
Principal amount outstanding											468,477,000
Gain (loss) on extinguishment of debt, net					$ (400,000)
Unsecured term loan due 2020
Debt instruments
Debt instrument, basis spread on variable rate					97.50%
Principal amount outstanding	900,000,000		900,000,000		$ 900,000,000						900,000,000
Unsecured Debt
Debt instruments
Line of credit facility, maximum borrowing capacity (up to)	$ 3,500,000,000.0		$ 3,500,000,000.0		$ 3,500,000,000.0
Debt instrument, face amount											20,000,000.0
3.125% Senior Notes due 2023
Debt instruments
Principal amount outstanding | CAD								CAD 400,000	CAD 400,000			CAD 0
Interest rate	1.55%		1.55%		1.55%			1.55%	3.125%
Public offering price as percent of par	100.335%		100.335%		100.335%			100.335%	99.343%
Proceeds from public offering sale of debt		$ 397,400,000
Gain (loss) on extinguishment of debt, net	$ 2,100,000
1.55% Senior Notes due 2016
Debt instruments
Repayments of debt	455,500,000
Principal amount outstanding	$ 94,530,000		$ 94,530,000		$ 94,530,000						$ 550,000,000
Interest rate	1.55%		1.55%		1.55%			1.55%			1.55%	1.55%
Mortgages
Debt instruments
Debt instrument, face amount	$ 180,500,000		$ 180,500,000		$ 180,500,000
Gain (loss) on extinguishment of debt, net					$ (300,000)
Weighted average maturity (in years)					2 years 8 months 5 days
Subsequent Event
Debt instruments
Derivative notional amount							$ 225,000,000
Derivative fixed interest rate							1.44%