SEGMENT INFORMATION - Income Statement (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Revenues:
Rental income	$ 354,512	$ 408,322	$ 1,126,934	$ 1,208,388
Income from loans and investments	18,666	30,164	62,203	66,819
Interest and other income	572	620	6,965	10,109
Total revenues	918,940	983,155	2,874,192	2,876,746
Less:
Interest and other income	572	620	6,965	10,109
Property-level operating expenses	494,985	461,493	1,474,558	1,327,898
Office building services costs	557	627	1,827	1,775
Segment NOI	422,826	520,415	1,390,842	1,536,964
Interest and other income	572	620	6,965	10,109
Interest expense	(115,505)	(113,967)	(355,333)	(334,955)
Depreciation and amortization	(249,366)	(234,603)	(847,797)	(696,710)
General, administrative and professional fees	(34,228)	(40,530)	(106,747)	(124,369)
Loss on extinguishment of debt, net	(7,386)	(37,434)	(7,386)	(41,861)
Merger-related expenses and deal costs	(11,325)	(4,304)	(26,129)	(11,084)
Other	(3,534)	(2,164)	(10,624)	9,294
Income Loss from Equity Method Investments and Remeasurement Gain on Equity Interest in Acquiree		854
Income (loss) from unconsolidated entities	865	854	(15,861)	(2,621)
Gain on real estate dispositions	12,622	36	240,101	24,633
Income tax benefit (expense)	3,195	(2,005)	95,855	57,004
Income from continuing operations	13,737	86,918	329,232	426,404
Net income	13,737	86,918	329,232	426,404
Net income attributable to noncontrolling interests	986	1,659	534	4,831
Net income attributable to common stockholders	12,751	85,259	328,698	421,573
Financing receivable, Change in allowance	(4,999)		(34,654)
Resident fees and services
Revenues:
Other revenues	541,322	541,090	1,667,421	1,583,262
Office building and other services revenue
Revenues:
Other revenues	3,868	2,959	10,669	8,168
Operating Segments
Less:
Office building services costs			1,827
Operating Segments | Triple-Net Leased Properties
Revenues:
Rental income	156,136	193,383	527,238	589,833
Income from loans and investments	0	0	0	0
Interest and other income	0	0	0	0
Total revenues	156,136	193,383	527,238	589,833
Less:
Interest and other income	0	0	0	0
Property-level operating expenses	5,398	6,338	17,004	20,092
Office building services costs	0	0	0	0
Segment NOI	150,738	187,045	510,234	569,741
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	541,322	541,090	1,667,421	1,583,262
Less:
Interest and other income	0	0	0	0
Property-level operating expenses	422,653	388,011	1,265,362	1,115,834
Office building services costs	0	0	0	0
Segment NOI	118,669	153,079	402,059	467,428
Interest and other income	0	0	0	0
Operating Segments | Senior Living Operations | Resident fees and services
Revenues:
Other revenues	541,322	541,090	1,667,421	1,583,262
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	198,376	214,939	599,696	618,555
Income from loans and investments	0	0	0	0
Interest and other income	0	0	0	0
Total revenues	200,816	216,998	606,567	624,240
Less:
Interest and other income	0	0	0	0
Property-level operating expenses	66,934	67,144	192,192	191,972
Office building services costs	557	627		1,775
Segment NOI	133,325	149,227	412,548	430,493
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	2,440	2,059	6,871	5,685
All Other
Revenues:
Rental income	0	0	0	0
Income from loans and investments	18,666	30,164	62,203	66,819
Interest and other income	572	620	6,965	10,109
Total revenues	20,666	31,684	72,966	79,411
Less:
Interest and other income	572	620	6,965	10,109
Property-level operating expenses	0	0	0	0
Office building services costs	0	0	0	0
Segment NOI	20,094	31,064	66,001	69,302
Interest and other income	572	620	6,965	10,109
All Other | Resident fees and services
Revenues:
Other revenues	0	0	0	0
All Other | Office building and other services revenue
Revenues:
Other revenues	$ 1,428	$ 900	$ 3,798	$ 2,483