SENIOR NOTES PAYABLE AND OTHER DEBT - Indebtedness of Maturities (Details) $ in Thousands	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2016 GBP (£)	Sep. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2017	$ 24,812
2018	922,703
2019	1,348,256
2020	1,449,721
2021	1,322,293
Thereafter	6,443,833
Total maturities	11,511,618	$ 11,190,914
Unrestricted cash and cash equivalents	85,063	286,707		$ 89,279	$ 53,023
Amount outstanding, net of cash and cash equivalents	453,800
Principal Amount Due at Maturity
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2017	18,539
2018	901,879
2019	1,333,378
2020	1,437,725
2021	772,837
Thereafter	6,327,024
Total maturities	10,791,382
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	538,911
Thereafter	0
Total maturities	538,911	$ 146,538	£ 0
Scheduled Periodic Amortization
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2017	6,273
2018	20,824
2019	14,878
2020	11,996
2021	10,545
Thereafter	116,809
Total maturities	$ 181,325
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
Interest rate	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
Interest rate	6.59%