SENIOR NOTES PAYABLE AND OTHER DEBT - Indebtedness of Maturities (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014	Dec. 31, 2013
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2015	$ 489,444
2016	900,869
2017	852,328
2018	1,709,225
2019	2,276,215
Thereafter	5,264,192
Total maturities	11,492,273	$ 10,872,371
Unrestricted cash and cash equivalents	60,532	55,348	$ 86,635	$ 94,816
Cash held in exchange escrow account	124,300
Amount outstanding, net of cash and cash equivalents	399,000
Principal Amount Due at Maturity
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2015	467,218
2016	861,817
2017	822,764
2018	1,101,879
2019	2,260,305
Thereafter	5,109,516
Total maturities	10,623,499
Unsecured Revolving Credit Facility
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2015	0
2016	0
2017	0
2018	583,765
2019	0
Thereafter	0
Total maturities	583,765	$ 919,099
Scheduled Periodic Amortization
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2015	22,226
2016	39,052
2017	29,564
2018	23,581
2019	15,910
Thereafter	154,676
Total maturities	285,009
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%