SEGMENT INFORMATION - Income Statement (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Revenues:
Rental income	$ 359,611	$ 369,165	$ 716,951	$ 772,422
Income from loans and investments	17,665	19,491	36,675	43,537
Interest and other income	585	1,540	926	6,393
Revenues	919,194	943,198	1,829,485	1,955,252
Less:
Interest and other income	585	1,540	926	6,393
Property-level operating expenses	494,195	498,605	980,795	979,573
Office building services costs	658	543	1,276	1,270
Segment NOI	423,756	442,510	846,488	968,016
Interest and other income	585	1,540	926	6,393
Interest expense	(110,051)	(123,132)	(220,818)	(239,828)
Depreciation and amortization	(250,700)	(349,594)	(564,848)	(598,431)
General, administrative and professional fees	(30,588)	(28,080)	(70,897)	(68,540)
Gain on extinguishment of debt, net	74	0	(27,016)	0
Merger-related expenses and deal costs	(721)	(6,586)	(5,338)	(14,804)
Other	13,490	(5,286)	22,918	(11,069)
Income Loss from Equity Method Investments and Remeasurement Gain on Equity Interest in Acquiree		(5,850)
Income (loss) from unconsolidated entities	4,767	(5,850)	4,517	(16,726)
Gain on real estate dispositions	41,258	1,254	43,791	227,479
Income tax (expense) benefit	(3,641)	(56,356)	(5,794)	92,660
Income (loss) from continuing operations	88,288	(159,235)	32,890	315,495
Net income	88,288	(159,235)	32,890	315,495
Net income (loss) attributable to noncontrolling interests	1,897	(2,065)	3,708	(452)
Net income attributable to common stockholders	86,391	(157,170)	29,182	315,947
Financing receivable, Change in allowance	59	(29,655)	8,961	(29,655)
Resident fees and services
Revenues:
Other revenues	535,952	549,329	1,064,602	1,126,099
Office building and other services revenue
Revenues:
Other revenues	5,381	3,673	10,331	6,801
Operating Segments
Less:
Office building services costs			1,276
Operating Segments | Triple-Net Leased Properties
Revenues:
Rental income	159,223	176,240	319,108	371,102
Income from loans and investments	0	0	0	0
Interest and other income	0	0	0	0
Revenues	159,223	176,240	319,108	371,102
Less:
Interest and other income	0	0	0	0
Property-level operating expenses	4,432	5,275	9,257	11,606
Office building services costs	0	0	0	0
Segment NOI	154,791	170,965	309,851	359,496
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
Revenues	535,952	549,329	1,064,602	1,126,099
Less:
Interest and other income	0	0	0	0
Property-level operating expenses	424,813	432,578	842,642	842,709
Office building services costs	0	0	0	0
Segment NOI	111,139	116,751	221,960	283,390
Interest and other income	0	0	0	0
Operating Segments | Senior Living Operations | Resident fees and services
Revenues:
Other revenues	535,952	549,329	1,064,602	1,126,099
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	200,388	192,925	397,843	401,320
Income from loans and investments	0	0	0	0
Interest and other income	0	0	0	0
Revenues	202,928	195,182	402,727	405,752
Less:
Interest and other income	0	0	0	0
Property-level operating expenses	64,950	60,752	128,896	125,258
Office building services costs	658	543	1,276	1,270
Segment NOI	137,320	133,887	272,555	279,224
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	2,540	2,257	4,884	4,432
All Other
Revenues:
Rental income	0	0	0	0
Income from loans and investments	17,665	19,491	36,675	43,537
Interest and other income	585	1,540	926	6,393
Revenues	21,091	22,447	43,048	52,299
Less:
Interest and other income	585	1,540	926	6,393
Property-level operating expenses	0	0	0	0
Office building services costs	0	0	0	0
Segment NOI	20,506	20,907	42,122	45,906
Interest and other income	585	1,540	926	6,393
All Other | Resident fees and services
Revenues:
Other revenues	0	0	0	0
All Other | Office building and other services revenue
Revenues:
Other revenues	$ 2,841	$ 1,416	$ 5,447	$ 2,369