SENIOR NOTES PAYABLE AND OTHER DEBT - Summary of Senior Notes Payables and Other Debt (Details) CAD in Thousands, $ in Thousands	Sep. 30, 2015 CAD	Sep. 30, 2015 USD ($)	Dec. 31, 2014 CAD	Dec. 31, 2014 USD ($)
Debt instruments
Principal amount outstanding		$ 11,333,707		$ 10,872,371
Deferred financing costs, net		(71,978)		(60,328)
Unamortized fair value adjustment		36,408		41,853
Unamortized discounts		(29,577)		(26,132)
Senior notes payable and other debt		11,268,560		10,827,764
Unsecured revolving credit facilities
Debt instruments
Principal amount outstanding		114,052		919,099
Unsecured revolving credit facilities | Canada, Dollars
Debt instruments
Principal amount outstanding | CAD	CAD 17,100		CAD 164,100
3.125% Senior Notes due 2015
Debt instruments
Principal amount outstanding		$ 0		$ 400,000
Interest rate (as a percent)	3.125%	3.125%	3.125%	3.125%
6% Senior Notes due 2015
Debt instruments
Principal amount outstanding		$ 0		$ 234,420
Interest rate (as a percent)	6.00%	6.00%	6.00%	6.00%
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
Debt instruments
Principal amount outstanding		$ 672,000
Unsecured term loan | Canada, Dollars
Debt instruments
Principal amount outstanding | CAD	CAD 93,400
Unsecured Term Loan Due 2018
Debt instruments
Principal amount outstanding		200,000		$ 200,000
Unsecured Term Loan Due 2019
Debt instruments
Principal amount outstanding		472,036		790,634
Unsecured term loan due 2020
Debt instruments
Principal amount outstanding		900,000		0
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 300,482		CAD 344,204
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%
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 187,800		CAD 0
Interest rate (as a percent)	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 187,800		CAD 215,128
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		$ 0
Interest rate (as a percent)	3.50%	3.50%
4.125% Senior Notes due 2026
Debt instruments
Principal amount outstanding		$ 500,000		0
Interest rate (as a percent)	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		$ 0
Interest rate (as a percent)	4.375%	4.375%
Mortgage loans and other
Debt instruments
Principal amount outstanding		$ 2,087,414		2,284,763
Mortgage Loans Held-for-Sale
Debt instruments
Principal amount outstanding		$ 48,300		$ 43,500