SENIOR NOTES PAYABLE AND OTHER DEBT - Indebtedness of Maturities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2018	$ 103,464
2019	838,243
2020	1,478,811
2021	1,580,621
2022	1,442,155
Thereafter	5,699,941
Total maturities	11,143,235	$ 11,365,633
Unrestricted cash and cash equivalents	92,543	81,355
Unsecured revolving credit facility borrowings outstanding, net of cash and cash equivalents	701,800
Principal Amount Due at Maturity
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2018	85,871
2019	820,716
2020	1,464,063
2021	772,867
2022	1,430,314
Thereafter	5,613,466
Total maturities	10,187,297
Unsecured Revolving Credit Facility
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2018	0
2019	0
2020	0
2021	794,368
2022	0
Thereafter	0
Total maturities	794,368	$ 535,832
Scheduled Periodic Amortization
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2018	17,593
2019	17,527
2020	14,748
2021	13,386
2022	11,841
Thereafter	86,475
Total maturities	161,570
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%