SEGMENT INFORMATION - Income Statement (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenues:
Rental income	$ 360,262	$ 399,498	$ 745,071	$ 794,720
Resident fees and services	518,989	460,243	1,033,742	924,431
Office building and other services revenue	4,289	3,179	7,617	6,585
Income from loans and investments	56,417	32,368	87,598	52,514
Interest and other income	2,347	202	11,981	683
Total revenues	942,304	895,490	1,886,009	1,778,933
Less:
Interest and other income	2,347	202	11,981	683
Property-level operating expenses	421,413	365,830	834,326	734,817
Office building services costs	534	552	649	1,290
Segment NOI	518,010	528,906	1,039,053	1,042,143
Income (loss) from unconsolidated entities	(6,371)	(106)	(47,110)	3,044
Segment profit	511,639	528,800	991,943	1,045,187
Interest and other income	2,347	202	11,981	683
Interest expense	(113,029)	(113,572)	(224,392)	(222,376)
Depreciation and amortization	(223,634)	(224,108)	(456,784)	(441,891)
General, administrative and professional fees	(36,656)	(33,282)	(73,830)	(67,243)
Gain on extinguishment of debt, net	93	(36)	(10,884)	(345)
Merger-related expenses and deal costs	(4,494)	(6,043)	(21,830)	(8,099)
Other	(3,527)	(1,848)	(6,647)	(3,036)
Income tax benefit	734	2,159	3,976	5,304
Income from continuing operations	133,473	152,272	213,533	308,184
Operating Segments | Triple-Net Leased Properties
Revenues:
Rental income	167,870	213,258	358,511	422,585
Resident fees and services	0	0	0	0
Office building and other services revenue	1,177	1,125	2,320	2,330
Income from loans and investments	0	0	0	0
Interest and other income	0	0	0	0
Total revenues	169,047	214,383	360,831	424,915
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	169,047	214,383	360,831	424,915
Income (loss) from unconsolidated entities	(4,694)	377	(43,349)	3,646
Segment profit	164,353	214,760	317,482	428,561
Interest and other income	0	0	0	0
Operating Segments | Senior Living Operations
Revenues:
Rental income	0	0	0	0
Resident fees and services	518,989	460,243	1,033,742	924,431
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	518,989	460,243	1,033,742	924,431
Less:
Interest and other income	0	0	0	0
Property-level operating expenses	361,112	308,625	713,332	620,698
Office building services costs	0	0	0	0
Segment NOI	157,877	151,618	320,410	303,733
Income (loss) from unconsolidated entities	(1,714)	(381)	(2,354)	(457)
Segment profit	156,163	151,237	318,056	303,276
Interest and other income	0	0	0	0
Operating Segments | Office Operations
Revenues:
Rental income	192,392	186,240	386,560	372,135
Resident fees and services	0	0	0	0
Office building and other services revenue	1,977	1,848	3,610	3,779
Income from loans and investments	0	0	0	0
Interest and other income	0	0	0	0
Total revenues	194,369	188,088	390,170	375,914
Less:
Interest and other income	0	0	0	0
Property-level operating expenses	60,301	57,205	120,994	114,119
Office building services costs	534	552	649	1,290
Segment NOI	133,534	130,331	268,527	260,505
Income (loss) from unconsolidated entities	885	298	265	633
Segment profit	134,419	130,629	268,792	261,138
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	1,135	206	1,687	476
Income from loans and investments	56,417	32,368	87,598	52,514
Interest and other income	2,347	202	11,981	683
Total revenues	59,899	32,776	101,266	53,673
Less:
Interest and other income	2,347	202	11,981	683
Property-level operating expenses	0	0	0	0
Office building services costs	0	0	0	0
Segment NOI	57,552	32,574	89,285	52,990
Income (loss) from unconsolidated entities	(848)	(400)	(1,672)	(778)
Segment profit	56,704	32,174	87,613	52,212
Interest and other income	$ 2,347	$ 202	$ 11,981	$ 683