SEGMENT INFORMATION - Income Statement (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) segment	Sep. 30, 2016 USD ($)
Segment Reporting [Abstract]
Number of reportable business segments | segment			3
Revenues:
Rental income	$ 401,876	$ 368,697	$ 1,196,596	$ 1,081,526
Resident fees and services	461,700	461,974	1,386,131	1,390,387
Office building and other services revenue	3,196	4,317	9,781	17,006
Income from loans and investments	32,985	31,566	85,499	78,098
Interest and other income	171	562	854	792
Total revenues	899,928	867,116	2,678,861	2,567,809
Less:
Interest and other income	171	562	854	792
Property-level operating expenses	376,207	361,117	1,111,024	1,069,294
Office building services costs	418	974	1,708	6,277
Segment NOI	523,132	504,463	1,565,275	1,491,446
Income (loss) from unconsolidated entities	750	931	3,794	2,151
Segment profit	523,882	505,394	1,569,069	1,493,597
Interest and other income	171	562	854	792
Interest expense	(113,869)	(105,063)	(336,245)	(312,001)
Depreciation and amortization	(213,407)	(208,387)	(655,298)	(666,735)
General, administrative and professional fees	(33,317)	(31,567)	(100,560)	(95,387)
Loss on extinguishment of debt, net	(511)	(383)	(856)	(3,165)
Merger-related expenses and deal costs	(804)	(16,217)	(8,903)	(25,073)
Other	(13,030)	(2,430)	(16,066)	(8,901)
Income tax benefit	7,815	8,537	13,119	28,507
Income from continuing operations	156,930	150,446	465,114	411,634
Operating Segments | Triple-Net Leased Properties
Revenues:
Rental income	212,370	210,424	634,955	635,030
Resident fees and services	0	0	0	0
Office building and other services revenue	1,125	1,246	3,455	3,676
Income from loans and investments	0	0	0	0
Interest and other income	0	0	0	0
Total revenues	213,495	211,670	638,410	638,706
Less:
Interest and other income	0	0	0	0
Property-level operating expenses	0	0	0	0
Office building services costs	0	0	0	0
Segment NOI	213,495	211,670	638,410	638,706
Income (loss) from unconsolidated entities	1,122	584	4,768	738
Segment profit	214,617	212,254	643,178	639,444
Interest and other income	0	0	0	0
Operating Segments | Senior Living Operations
Revenues:
Rental income	0	0	0	0
Resident fees and services	461,700	461,974	1,386,131	1,390,387
Office building and other services revenue	0	0	0	0
Income from loans and investments	0	0	0	0
Interest and other income	0	0	0	0
Total revenues	461,700	461,974	1,386,131	1,390,387
Less:
Interest and other income	0	0	0	0
Property-level operating expenses	315,598	312,145	936,296	932,675
Office building services costs	0	0	0	0
Segment NOI	146,102	149,829	449,835	457,712
Income (loss) from unconsolidated entities	300	75	(157)	732
Segment profit	146,402	149,904	449,678	458,444
Interest and other income	0	0	0	0
Operating Segments | Office Operations
Revenues:
Rental income	189,506	158,273	561,641	446,496
Resident fees and services	0	0	0	0
Office building and other services revenue	1,568	2,211	5,347	10,556
Income from loans and investments	0	0	0	0
Interest and other income	0	0	0	0
Total revenues	191,074	160,484	566,988	457,052
Less:
Interest and other income	0	0	0	0
Property-level operating expenses	60,609	48,972	174,728	136,619
Office building services costs	418	974	1,708	6,277
Segment NOI	130,047	110,538	390,552	314,156
Income (loss) from unconsolidated entities	(348)	238	284	301
Segment profit	129,699	110,776	390,836	314,457
Interest and other income	0	0	0	0
All Other
Revenues:
Rental income	0	0	0	0
Resident fees and services	0	0	0	0
Office building and other services revenue	503	860	979	2,774
Income from loans and investments	32,985	31,566	85,499	78,098
Interest and other income	171	562	854	792
Total revenues	33,659	32,988	87,332	81,664
Less:
Interest and other income	171	562	854	792
Property-level operating expenses	0	0	0	0
Office building services costs	0	0	0	0
Segment NOI	33,488	32,426	86,478	80,872
Income (loss) from unconsolidated entities	(324)	34	(1,101)	380
Segment profit	33,164	32,460	85,377	81,252
Interest and other income	$ 171	$ 562	$ 854	$ 792