SEGMENT INFORMATION - Income Statement (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Revenues:
Rental income	$ 398,570	$ 360,262	$ 800,066	$ 745,071
Income from loans and investments	19,529	56,417	36,655	87,598
Interest and other income	9,202	2,347	9,489	11,981
Total revenues	950,717	942,304	1,893,591	1,886,009
Less:
Interest and other income	9,202	2,347	9,489	11,981
Property-level operating expenses	435,901	421,413	866,405	834,326
Office building services costs	515	534	1,148	649
Segment NOI	505,099	518,010	1,016,549	1,039,053
Loss from unconsolidated entities	(2,529)	(6,371)	(3,475)	(47,110)
Segment profit	502,570	511,639	1,013,074	991,943
Interest and other income	9,202	2,347	9,489	11,981
Interest expense	(110,369)	(113,029)	(220,988)	(224,392)
Depreciation and amortization	(226,187)	(223,634)	(462,107)	(456,784)
General, administrative and professional fees	(43,079)	(36,656)	(83,839)	(73,830)
Loss on extinguishment of debt, net	4,022	(93)	4,427	10,884
Merger-related expenses and deal costs	(4,600)	(4,494)	(6,780)	(21,830)
Other	11,481	(3,527)	11,458	(6,647)
Gain on real estate dispositions	19,150	35,827	24,597	35,875
Income tax benefit	57,752	734	59,009	3,976
Income from continuing operations	211,898	169,300	339,486	249,408
Discontinued operations	0	0	0	(10)
Net income	211,898	169,300	339,486	249,398
Net income attributable to noncontrolling interests	1,369	2,781	3,172	4,176
Net income attributable to common stockholders	210,529	166,519	336,314	245,222
Resident Fees and Services
Revenues:
Other revenues	520,725	518,989	1,042,172	1,033,742
Office Building and Other Services Revenue
Revenues:
Other revenues	2,691	4,289	5,209	7,617
Operating Segments | Triple-Net Leased Properties
Revenues:
Rental income	196,382	167,870	396,450	358,511
Income from loans and investments	0	0	0	0
Interest and other income	0	0	0	0
Total revenues	196,382	169,047	396,450	360,831
Less:
Interest and other income	0	0	0	0
Property-level operating expenses	6,321	0	13,754	0
Office building services costs	0	0	0	0
Segment NOI	190,061	169,047	382,696	360,831
Loss from unconsolidated entities	(1,285)	(4,694)	(777)	(43,349)
Segment profit	188,776	164,353	381,919	317,482
Interest and other income	0	0	0	0
Operating Segments | Triple-Net Leased Properties | Resident Fees and Services
Revenues:
Other revenues	0	0	0	0
Operating Segments | Triple-Net Leased Properties | Office Building and Other Services Revenue
Revenues:
Other revenues	0	1,177	0	2,320
Operating Segments | Senior Living Operations
Revenues:
Rental income	0	0	0	0
Income from loans and investments	0	0	0	0
Interest and other income	0	0	0	0
Total revenues	520,725	518,989	1,042,172	1,033,742
Less:
Interest and other income	0	0	0	0
Property-level operating expenses	366,837	361,112	727,823	713,332
Office building services costs	0	0	0	0
Segment NOI	153,888	157,877	314,349	320,410
Loss from unconsolidated entities	(549)	(1,714)	(1,483)	(2,354)
Segment profit	153,339	156,163	312,866	318,056
Interest and other income	0	0	0	0
Operating Segments | Senior Living Operations | Resident Fees and Services
Revenues:
Other revenues	520,725	518,989	1,042,172	1,033,742
Operating Segments | Senior Living Operations | Office Building and Other Services Revenue
Revenues:
Other revenues	0	0	0	0
Operating Segments | Office Operations
Revenues:
Rental income	202,188	192,392	403,616	386,560
Income from loans and investments	0	0	0	0
Interest and other income	0	0	0	0
Total revenues	204,038	194,369	407,242	390,170
Less:
Interest and other income	0	0	0	0
Property-level operating expenses	62,743	60,301	124,828	120,994
Office building services costs	515	534	1,148	649
Segment NOI	140,780	133,534	281,266	268,527
Loss from unconsolidated entities	235	885	464	265
Segment profit	141,015	134,419	281,730	268,792
Interest and other income	0	0	0	0
Operating Segments | Office Operations | Resident Fees and Services
Revenues:
Other revenues	0	0	0	0
Operating Segments | Office Operations | Office Building and Other Services Revenue
Revenues:
Other revenues	1,850	1,977	3,626	3,610
All Other
Revenues:
Rental income	0	0	0	0
Income from loans and investments	19,529	56,417	36,655	87,598
Interest and other income	9,202	2,347	9,489	11,981
Total revenues	29,572	59,899	47,727	101,266
Less:
Interest and other income	9,202	2,347	9,489	11,981
Property-level operating expenses	0	0	0	0
Office building services costs	0	0	0	0
Segment NOI	20,370	57,552	38,238	89,285
Loss from unconsolidated entities	(930)	(848)	(1,679)	(1,672)
Segment profit	19,440	56,704	36,559	87,613
Interest and other income	9,202	2,347	9,489	11,981
All Other | Resident Fees and Services
Revenues:
Other revenues	0	0	0	0
All Other | Office Building and Other Services Revenue
Revenues:
Other revenues	$ 841	$ 1,135	$ 1,583	$ 1,687