SENIOR NOTES PAYABLE AND OTHER DEBT - Narrative (Details) CAD in Millions		1 Months Ended				3 Months Ended		6 Months Ended
	Apr. 25, 2017 USD ($) period	Jun. 30, 2017 CAD	May 31, 2017 CAD	Apr. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 GBP (£)	Jun. 30, 2017 CAD	Apr. 03, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2016 GBP (£)	Dec. 31, 2016 CAD
Debt instruments
Principal amount outstanding						$ 11,992,233,000		$ 11,992,233,000					$ 11,190,914,000
Gain on repayment of debt						$ (36,000)	$ (2,468,000)	$ (345,000)	$ (2,782,000)
Derivative gain					$ 800,000
Forward Contracts
Debt instruments
Derivative notional amount												$ 275,000,000
Debt, weighted average interest rate												2.33%
Interest Rate Swap
Debt instruments
Derivative notional amount					400,000,000
Derivative fixed interest rate						3.10%		3.10%		3.10%	3.10%
Swap
Debt instruments
Derivative notional amount						$ 125,000,000		$ 125,000,000
LIBOR | Interest Rate Swap
Debt instruments
Debt, weighted average interest rate						0.98%		0.98%		0.98%	0.98%
LIBOR | Swap
Debt instruments
Debt, weighted average interest rate						2.1832%		2.1832%		2.1832%	2.1832%
Unsecured revolving credit facilities
Debt instruments
Principal amount outstanding						$ 516,455,000		$ 516,455,000		£ 30,400,000	CAD 33.1		146,538,000	£ 0	CAD 146.5
Unsecured Term Loan Due 2018 [Member]
Debt instruments
Principal amount outstanding						200,000,000		200,000,000
Unsecured Term Loan Due 2018
Debt instruments
Principal amount outstanding						200,000,000		200,000,000					200,000,000
Unsecured term loan due 2019
Debt instruments
Principal amount outstanding						278,586,000		$ 278,586,000					371,215,000		CAD 92.6
Repayments of debt | CAD		CAD 124.4
Unsecured term loan due 2019 | LIBOR
Debt instruments
Debt instrument, basis spread on variable rate	1.05%
Unsecured term loan due 2020
Debt instruments
Debt instrument, basis spread on variable rate								0.975%
Principal amount outstanding						900,000,000		$ 900,000,000					900,000,000
Mortgage Loans [Member]
Debt instruments
Debt instrument, term of loan								1 year 8 months
Repayments of debt								$ 237,000,000
Gain on repayment of debt								400,000
3.100% Senior Notes due 2023
Debt instruments
Principal amount outstanding					$ 400,000,000	$ 400,000,000		$ 400,000,000					0
Interest rate					3.10%	3.10%		3.10%		3.10%	3.10%
Public offering price as percent of par					99.28%
Proceeds from public offering sale of debt					$ 397,120,000
3.850% Senior Notes due 2027
Debt instruments
Principal amount outstanding					$ 400,000,000	$ 400,000,000		$ 400,000,000					0
Interest rate					3.85%	3.85%		3.85%		3.85%	3.85%
Public offering price as percent of par					99.196%
Proceeds from public offering sale of debt					$ 396,800,000
1.250% Senior Notes due 2017
Debt instruments
Principal amount outstanding						$ 0		$ 0					$ 300,000,000
Interest rate				1.25%		1.25%		1.25%		1.25%	1.25%
Repayments of debt				$ 300,000,000
2.55% Senior Notes, Series D due 2023 (3)
Debt instruments
Principal amount outstanding | CAD			CAD 275.0
Interest rate			2.55%			2.55%		2.55%		2.55%	2.55%
Public offering price as percent of par			99.954%
Proceeds from public offering sale of debt | CAD			CAD 274.9
Revolving Credit Facility
Debt instruments
Line of Credit Facility, Fair Value of Amount Outstanding						$ 516,500,000		$ 516,500,000
Additional periods | period	2
Debt instrument, additional period	6 months
Letters of credit outstanding						14,500,000		14,500,000
Unused borrowing capacity available						$ 2,500,000,000		$ 2,500,000,000
Revolving Credit Facility | LIBOR
Debt instruments
Debt instrument, basis spread on variable rate	0.875%
Revolving Credit Facility | Unsecured revolving credit facilities
Debt instruments
Line of credit facility, maximum borrowing capacity (up to)	$ 3,000,000,000.0
Revolving Credit Facility | Accordion Feature
Debt instruments
Line of credit facility, maximum borrowing capacity (up to)	$ 3,750,000,000.00
Previous Revolving Credit Facility [Member] | LIBOR
Debt instruments
Debt instrument, basis spread on variable rate	1.00%
Previous Revolving Credit Facility [Member] | Unsecured revolving credit facilities
Debt instruments
Line of credit facility, maximum borrowing capacity (up to)	$ 2,000,000,000.0