SEGMENT INFORMATION - Income Statement (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2017 USD ($) segment	Mar. 31, 2016 USD ($)
Segment Reporting [Abstract]
Number of reportable business segments | segment	3
Revenues:
Rental income	$ 395,222	$ 358,623
Resident fees and services	464,188	463,976
Office building and other services revenue	3,406	7,185
Income from loans and investments	20,146	22,386
Interest and other income	481	119
Total revenues	883,443	852,289
Less:
Interest and other income	481	119
Property-level operating expenses	368,987	356,222
Office building services costs	738	3,451
Segment NOI	513,237	492,497
Income (loss) from unconsolidated entities	3,150	(198)
Segment profit	516,387	492,299
Interest and other income	481	119
Interest expense	(108,804)	(103,273)
Depreciation and amortization	(217,783)	(236,387)
General, administrative and professional fees	(33,961)	(31,726)
Loss on extinguishment of debt, net	(309)	(314)
Merger-related expenses and deal costs	(2,056)	(1,632)
Other	(1,188)	(4,168)
Income tax benefit	3,145	8,421
Income from continuing operations	155,912	123,339
Operating Segments | Triple-Net Leased Properties
Revenues:
Rental income	209,327	214,487
Resident fees and services	0	0
Office building and other services revenue	1,205	1,199
Income from loans and investments	0	0
Interest and other income	0	0
Total revenues	210,532	215,686
Less:
Interest and other income	0	0
Property-level operating expenses	0	0
Office building services costs	0	0
Segment NOI	210,532	215,686
Income (loss) from unconsolidated entities	3,269	(671)
Segment profit	213,801	215,015
Interest and other income	0	0
Operating Segments | Senior Living Operations
Revenues:
Rental income	0	0
Resident fees and services	464,188	463,976
Office building and other services revenue	0	0
Income from loans and investments	0	0
Interest and other income	0	0
Total revenues	464,188	463,976
Less:
Interest and other income	0	0
Property-level operating expenses	312,073	312,541
Office building services costs	0	0
Segment NOI	152,115	151,435
Income (loss) from unconsolidated entities	(76)	337
Segment profit	152,039	151,772
Interest and other income	0	0
Operating Segments | Office Operations
Revenues:
Rental income	185,895	144,136
Resident fees and services	0	0
Office building and other services revenue	1,931	4,976
Income from loans and investments	0	0
Interest and other income	0	0
Total revenues	187,826	149,112
Less:
Interest and other income	0	0
Property-level operating expenses	56,914	43,681
Office building services costs	738	3,451
Segment NOI	130,174	101,980
Income (loss) from unconsolidated entities	335	(126)
Segment profit	130,509	101,854
Interest and other income	0	0
All Other
Revenues:
Rental income	0	0
Resident fees and services	0	0
Office building and other services revenue	270	1,010
Income from loans and investments	20,146	22,386
Interest and other income	481	119
Total revenues	20,897	23,515
Less:
Interest and other income	481	119
Property-level operating expenses	0	0
Office building services costs	0	0
Segment NOI	20,416	23,396
Income (loss) from unconsolidated entities	(378)	262
Segment profit	20,038	23,658
Interest and other income	$ 481	$ 119