SENIOR NOTES PAYABLE AND OTHER DEBT - Indebtedness of Maturities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2018	$ 41,745
2019	734,704
2020	1,490,608
2021	1,089,074
2022	1,456,330
Thereafter	5,692,516
Total maturities	10,504,977	$ 11,365,633
Unrestricted cash and cash equivalents	93,684	81,355
Unsecured revolving credit facility borrowings outstanding, net of cash and cash equivalents	209,100
Principal Amount Due at Maturity
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2018	30,158
2019	717,177
2020	1,475,860
2021	772,868
2022	1,444,489
Thereafter	5,606,042
Total maturities	10,046,594
Unsecured Revolving Credit Facility
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2018	0
2019	0
2020	0
2021	302,820
2022	0
Thereafter	0
Total maturities	302,820	$ 535,832
Scheduled Periodic Amortization
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2018	11,587
2019	17,527
2020	14,748
2021	13,386
2022	11,841
Thereafter	86,474
Total maturities	155,563
Senior Notes Included in Thereafter Maturities | 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%
Senior Notes Included in Thereafter Maturities | 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
Interest rate	6.59%