SENIOR NOTES PAYABLE AND OTHER DEBT - Summary of Senior Notes Payables and Other Debt (Details) CAD in Thousands, $ in Thousands	Mar. 31, 2016 CAD	Mar. 31, 2016 USD ($)	Feb. 29, 2016	Dec. 31, 2015 CAD	Dec. 31, 2015 USD ($)
Debt instruments
Principal amount outstanding		$ 11,310,026			$ 11,271,020
Deferred financing costs, net		(65,555)			(69,121)
Unamortized fair value adjustment		30,751			33,570
Unamortized discounts		(27,492)			(28,473)
Senior notes payable and other debt		11,247,730			11,206,996
Interest rate (as a percent)			1.132%
Unsecured revolving credit facilities
Debt instruments
Principal amount outstanding		324,488			180,683
Unsecured revolving credit facilities | Canada, Dollars
Debt instruments
Principal amount outstanding | CAD	CAD 163,500			CAD 9,700
1.55% Senior Notes due 2016
Debt instruments
Principal amount outstanding		$ 550,000			$ 550,000
Interest rate (as a percent)	1.55%	1.55%		1.55%	1.55%
1.250% Senior Notes due 2017
Debt instruments
Principal amount outstanding		$ 300,000			$ 300,000
Interest rate (as a percent)	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 (as a percent)	2.00%	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		474,249			468,477
4.00% Senior Notes due 2019
Debt instruments
Principal amount outstanding		$ 600,000			$ 600,000
Interest rate (as a percent)	4.00%	4.00%		4.00%	4.00%
3.00% Senior Notes, Series A due 2019
Debt instruments
Principal amount outstanding | CAD	CAD 307,598			CAD 289,038
Interest rate (as a percent)	3.00%	3.00%		3.00%	3.00%
2.700% Senior Notes due 2020
Debt instruments
Principal amount outstanding		$ 500,000			$ 500,000
Interest rate (as a percent)	2.70%	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 (as a percent)	4.75%	4.75%		4.75%	4.75%
4.25% Senior Notes due 2022
Debt instruments
Principal amount outstanding		$ 600,000			$ 600,000
Interest rate (as a percent)	4.25%	4.25%		4.25%	4.25%
3.25% Senior Notes due 2022
Debt instruments
Principal amount outstanding		$ 500,000			$ 500,000
Interest rate (as a percent)	3.25%	3.25%		3.25%	3.25%
3.300% Senior Notes due 2022
Debt instruments
Principal amount outstanding | CAD	CAD 192,249			CAD 180,649
Interest rate (as a percent)	3.30%	3.30%		3.30%	3.30%
3.750% Senior Notes due 2024
Debt instruments
Principal amount outstanding		$ 400,000			$ 400,000
Interest rate (as a percent)	3.75%	3.75%		3.75%	3.75%
4.125% Senior Notes, Series B due 2024
Debt instruments
Principal amount outstanding | CAD	CAD 192,249			CAD 180,649
Interest rate (as a percent)	4.125%	4.125%		4.125%	4.125%
3.500% Senior Notes due 2025
Debt instruments
Principal amount outstanding		$ 600,000			$ 600,000
Interest rate (as a percent)	3.50%	3.50%		3.50%	3.50%
4.125% Senior Notes due 2026
Debt instruments
Principal amount outstanding		$ 500,000			$ 500,000
Interest rate (as a percent)	4.125%	4.125%		4.125%	4.125%
6.90% Senior Notes due 2037
Debt instruments
Principal amount outstanding		$ 52,400			$ 52,400
Interest rate (as a percent)	6.90%	6.90%		6.90%	6.90%
6.59% Senior Notes due 2038
Debt instruments
Principal amount outstanding		$ 22,973			$ 22,973
Interest rate (as a percent)	6.59%	6.59%		6.59%	6.59%
5.45% Senior Notes due 2043
Debt instruments
Principal amount outstanding		$ 258,750			$ 258,750
Interest rate (as a percent)	5.45%	5.45%		5.45%	5.45%
5.70% Senior Notes due 2043
Debt instruments
Principal amount outstanding		$ 300,000			$ 300,000
Interest rate (as a percent)	5.70%	5.70%		5.70%	5.70%
4.375% Senior Notes due 2045
Debt instruments
Principal amount outstanding		$ 300,000			$ 300,000
Interest rate (as a percent)	4.375%	4.375%		4.375%	4.375%
Mortgage loans and other
Debt instruments
Principal amount outstanding		$ 1,835,070			$ 1,987,401
Unsecured term loan
Debt instruments
Principal amount outstanding		674,200
Unsecured term loan | Canada, Dollars
Debt instruments
Principal amount outstanding | CAD	CAD 95,700
Mortgage Loans Held-for-Sale
Debt instruments
Principal amount outstanding		$ 11,100			$ 22,900