SENIOR NOTES PAYABLE AND OTHER DEBT - Indebtedness of Maturities (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2015	$ 18,109
2016	648,443
2017	852,257
2018	1,239,434
2019	1,936,879
Thereafter	6,638,585
Total maturities	11,333,707	$ 10,872,371
Unrestricted cash and cash equivalents	65,231	55,348	$ 64,595	$ 94,816
Amount outstanding, net of cash and cash equivalents	48,800
Principal Amount Due at Maturity
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2015	9,057
2016	615,077
2017	822,764
2018	1,101,879
2019	1,920,978
Thereafter	6,501,581
Total maturities	10,971,336
Unsecured Revolving Credit Facility
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2015	0
2016	0
2017	0
2018	114,052
2019	0
Thereafter	0
Total maturities	114,052	$ 919,099
Scheduled Periodic Amortization
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2015	9,052
2016	33,366
2017	29,493
2018	23,503
2019	15,901
Thereafter	137,004
Total maturities	248,319
Senior Notes | 6.90% Senior Notes due 2037
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
Thereafter	$ 52,400
Interest rate (as a percent)	6.90%
Senior Notes | 6.59% Senior Notes due 2038
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
Thereafter	$ 23,000
Interest rate (as a percent)	6.59%