SEGMENT INFORMATION - Income Statement (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Revenues:
Rental income	$ 369,165	$ 398,570	$ 772,422	$ 800,066
Income from loans and investments	19,491	19,529	43,537	36,655
Interest and other income	1,540	9,202	6,393	9,489
Total revenues	943,198	950,717	1,955,252	1,893,591
Less:
Interest and other income	1,540	9,202	6,393	9,489
Property-level operating expenses	498,605	435,901	979,573	866,405
Office building services costs	543	515	1,270	1,148
Segment NOI	442,510	505,099	968,016	1,016,549
Interest and other income	1,540	9,202	6,393	9,489
Interest expense	(123,132)	(110,369)	(239,828)	(220,988)
Depreciation and amortization	(349,594)	(226,187)	(598,431)	(462,107)
General, administrative and professional fees	(29,984)	(43,079)	(72,519)	(83,839)
Loss on extinguishment of debt, net	0	(4,022)	0	(4,427)
Merger-related expenses and deal costs	(6,586)	(4,600)	(14,804)	(6,780)
Other	(3,382)	11,481	(7,090)	11,458
Income Loss from Equity Method Investments and Remeasurement Gain on Equity Interest in Acquiree		(2,529)
Loss from unconsolidated entities	(5,850)	(2,529)	(16,726)	(3,475)
Gain on real estate dispositions	1,254	19,150	227,479	24,597
Income tax (expense) benefit	(56,356)	57,752	92,660	59,009
(Loss) income from continuing operations	(159,235)	211,898	315,495	339,486
Net income	(159,235)	211,898	315,495	339,486
Net (loss) income attributable to noncontrolling interests	(2,065)	1,369	(452)	3,172
Net income attributable to common stockholders	(157,170)	210,529	315,947	336,314
Financing receivable, Change in allowance	(29,655)		(29,655)
Resident Fees and Services
Revenues:
Other revenues	549,329	520,725	1,126,099	1,042,172
Office Building and Other Services Revenue
Revenues:
Other revenues	3,673	2,691	6,801	5,209
Operating Segments | Triple-Net Leased Properties
Revenues:
Rental income	176,240	196,382	371,102	396,450
Income from loans and investments	0	0	0	0
Interest and other income	0	0	0	0
Total revenues	176,240	196,382	371,102	396,450
Less:
Interest and other income	0	0	0	0
Property-level operating expenses	5,275	6,321	11,606	13,754
Office building services costs	0	0	0	0
Segment NOI	170,965	190,061	359,496	382,696
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	0	0	0
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	549,329	520,725	1,126,099	1,042,172
Less:
Interest and other income	0	0	0	0
Property-level operating expenses	432,578	366,837	842,709	727,823
Office building services costs	0	0	0	0
Segment NOI	116,751	153,888	283,390	314,349
Interest and other income	0	0	0	0
Operating Segments | Senior Living Operations | Resident Fees and Services
Revenues:
Other revenues	549,329	520,725	1,126,099	1,042,172
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	192,925	202,188	401,320	403,616
Income from loans and investments	0	0	0	0
Interest and other income	0	0	0	0
Total revenues	195,182	204,038	405,752	407,242
Less:
Interest and other income	0	0	0	0
Property-level operating expenses	60,752	62,743	125,258	124,828
Office building services costs	543	515	1,270	1,148
Segment NOI	133,887	140,780	279,224	281,266
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	2,257	1,850	4,432	3,626
All Other
Revenues:
Rental income	0	0	0	0
Income from loans and investments	19,491	19,529	43,537	36,655
Interest and other income	1,540	9,202	6,393	9,489
Total revenues	22,447	29,572	52,299	47,727
Less:
Interest and other income	1,540	9,202	6,393	9,489
Property-level operating expenses	0	0	0	0
Office building services costs	0	0	0	0
Segment NOI	20,907	20,370	45,906	38,238
Interest and other income	1,540	9,202	6,393	9,489
All Other | Resident Fees and Services
Revenues:
Other revenues	0	0	0	0
All Other | Office Building and Other Services Revenue
Revenues:
Other revenues	$ 1,416	$ 841	$ 2,369	$ 1,583