SENIOR NOTES PAYABLE AND OTHER DEBT - Indebtedness of Maturities (Details) $ in Thousands, CAD in Millions	Jun. 30, 2017 USD ($)	Jun. 30, 2017 GBP (£)	Jun. 30, 2017 CAD	Dec. 31, 2016 USD ($)	Dec. 31, 2016 GBP (£)	Dec. 31, 2016 CAD	Jun. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2017	$ 115,172
2018	1,122,438
2019	1,614,309
2020	1,434,244
2021	1,299,521
Thereafter	6,406,549
Total maturities	11,992,233			$ 11,190,914
Unrestricted cash and cash equivalents	103,353			286,707			$ 57,322	$ 53,023
Amount outstanding, net of cash and cash equivalents	413,100
Principal Amount Due at Maturity
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2017	103,561
2018	1,101,879
2019	1,599,713
2020	1,422,547
2021	772,838
Thereafter	6,291,984
Total maturities	11,292,522
Principal Amount Due at Maturity | 6.90% Senior Notes due 2037, subject to repurchase at option of holders
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
Total maturities	52,400
Principal Amount Due at Maturity | 6.59% Senior Notes due 2038, subject to repurchase at option of holders
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
Total maturities	23,000
Unsecured Revolving Credit Facility
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2017	0
2018	0
2019	0
2020	0
2021	516,455
Thereafter	0
Total maturities	516,455	£ 30,400,000	CAD 33.1	$ 146,538	£ 0	CAD 146.5
Scheduled Periodic Amortization
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2017	11,611
2018	20,559
2019	14,596
2020	11,697
2021	10,228
Thereafter	114,565
Total maturities	183,256
Senior Notes | 6.90% Senior Notes due 2037, subject to repurchase at option of holders
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
Total maturities	$ 52,400
Interest rate	6.90%	6.90%	6.90%
Senior Notes | 6.59% Senior Notes due 2038, subject to repurchase at option of holders
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
Total maturities	$ 22,973
Interest rate	6.59%	6.59%	6.59%