SEGMENT INFORMATION - Income Statement (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Revenues:
Rental income	$ 383,052	$ 354,512	$ 1,100,003	$ 1,126,934
Income from loans and investments	28,729	18,666	65,404	62,203
Interest and other income	417	572	1,343	6,965
Revenues	976,078	918,940	2,805,563	2,874,192
Less:
Interest and other income	417	572	1,343	6,965
Property-level operating expenses	523,328	494,985	1,504,123	1,474,558
Office building services costs	522	557	1,798	1,827
Segment NOI	451,811	422,826	1,298,299	1,390,842
Interest and other income	417	572	1,343	6,965
Interest expense	(108,816)	(115,505)	(329,634)	(355,333)
Depreciation and amortization	(313,596)	(249,366)	(878,444)	(847,797)
General, administrative and professional fees	(30,259)	(32,081)	(101,156)	(100,621)
Loss on extinguishment of debt, net	(29,792)	(7,386)	(56,808)	(7,386)
Merger-related expenses and deal costs	(22,662)	(11,325)	(28,000)	(26,129)
Other	(33,673)	(5,681)	(10,755)	(16,750)
Income Loss from Equity Method Investments and Remeasurement Gain on Equity Interest in Acquiree		865
Income (loss) from unconsolidated entities	2,772	865	7,289	(15,861)
Gain on real estate dispositions	150,292	12,622	194,083	240,101
Income tax (expense) benefit	(3,780)	3,195	(9,574)	95,855
Income from continuing operations	62,774	13,737	95,664	329,232
Net income	62,774	13,737	95,664	329,232
Net income attributable to noncontrolling interests	2,094	986	5,802	534
Net income attributable to common stockholders	60,680	12,751	89,862	328,698
Financing receivable, Change in allowance	60	(4,999)	9,021	(34,654)
Resident fees and services
Revenues:
Other revenues	558,039	541,322	1,622,641	1,667,421
Office building and other services revenue
Revenues:
Other revenues	5,841	3,868	16,172	10,669
Operating Segments
Less:
Office building services costs			1,798
Operating Segments | Triple-Net Leased Properties
Revenues:
Rental income	181,379	156,136	500,487	527,238
Income from loans and investments	0	0	0	0
Interest and other income	0	0	0	0
Revenues	181,379	156,136	500,487	527,238
Less:
Interest and other income	0	0	0	0
Property-level operating expenses	3,268	5,398	12,525	17,004
Office building services costs	0	0	0	0
Segment NOI	178,111	150,738	487,962	510,234
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	558,039	541,322	1,622,641	1,667,421
Less:
Interest and other income	0	0	0	0
Property-level operating expenses	453,659	422,653	1,296,301	1,265,362
Office building services costs	0	0	0	0
Segment NOI	104,380	118,669	326,340	402,059
Interest and other income	0	0	0	0
Operating Segments | Senior Living Operations | Resident fees and services
Revenues:
Other revenues	558,039	541,322	1,622,641	1,667,421
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	201,673	198,376	599,516	599,696
Income from loans and investments	0	0	0	0
Interest and other income	0	0	0	0
Revenues	204,545	200,816	607,272	606,567
Less:
Interest and other income	0	0	0	0
Property-level operating expenses	66,401	66,934	195,297	192,192
Office building services costs	522	557	1,798	1,827
Segment NOI	137,622	133,325	410,177	412,548
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	2,872	2,440	7,756	6,871
All Other
Revenues:
Rental income	0	0	0	0
Income from loans and investments	28,729	18,666	65,404	62,203
Interest and other income	417	572	1,343	6,965
Revenues	32,115	20,666	75,163	72,966
Less:
Interest and other income	417	572	1,343	6,965
Property-level operating expenses	0	0	0	0
Office building services costs	0	0	0	0
Segment NOI	31,698	20,094	73,820	66,001
Interest and other income	417	572	1,343	6,965
All Other | Resident fees and services
Revenues:
Other revenues	0	0	0	0
All Other | Office building and other services revenue
Revenues:
Other revenues	$ 2,969	$ 1,428	$ 8,416	$ 3,798