SEGMENT INFORMATION - Income Statement (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Revenues:
Rental income	$ 403,257	$ 401,496
Income from loans and investments	24,046	17,126
Interest and other income	4,853	287
Total revenues	1,012,054	942,874
Less:
Interest and other income	4,853	287
Property-level operating expenses	480,968	430,504
Office building services costs	727	633
Segment NOI	525,506	511,450
Interest and other income	4,853	287
Interest expense	(116,696)	(110,619)
Depreciation and amortization	(248,837)	(235,920)
General, administrative and professional fees	(42,535)	(40,760)
Loss on extinguishment of debt, net	0	405
Merger-related expenses and deal costs	(8,218)	(2,180)
Other	(3,708)	(23)
Income Loss from Equity Method Investments and Remeasurement Gain on Equity Interest in Acquiree		(946)
Loss from unconsolidated entities	(10,876)	(946)
Gain on real estate dispositions	226,225	5,447
Income tax benefit	149,016	1,257
Income from continuing operations	474,730	127,588
Net income	474,730	127,588
Net income attributable to noncontrolling interests	1,613	1,803
Net income attributable to common stockholders	473,117	125,785
Resident Fees and Services
Revenues:
Other revenues	576,770	521,447
Office Building and Other Services Revenue
Revenues:
Other revenues	3,128	2,518
Operating Segments | Triple-Net Leased Properties
Revenues:
Rental income	194,862	200,068
Income from loans and investments	0	0
Interest and other income	0	0
Total revenues	194,862	200,068
Less:
Interest and other income	0	0
Property-level operating expenses	6,331	7,433
Office building services costs	0	0
Segment NOI	188,531	192,635
Interest and other income	0	0
Operating Segments | Triple-Net Leased Properties | Resident Fees and Services
Revenues:
Other revenues	0	0
Operating Segments | Triple-Net Leased Properties | Office Building and Other Services Revenue
Revenues:
Other revenues	0	0
Operating Segments | Senior Living Operations
Revenues:
Rental income	0	0
Income from loans and investments	0	0
Interest and other income	0	0
Total revenues	576,770	521,447
Less:
Interest and other income	0	0
Property-level operating expenses	410,131	360,986
Office building services costs	0	0
Segment NOI	166,639	160,461
Interest and other income	0	0
Operating Segments | Senior Living Operations | Resident Fees and Services
Revenues:
Other revenues	576,770	521,447
Operating Segments | Senior Living Operations | Office Building and Other Services Revenue
Revenues:
Other revenues	0	0
Operating Segments | Office Operations
Revenues:
Rental income	208,395	201,428
Income from loans and investments	0	0
Interest and other income	0	0
Total revenues	210,569	203,203
Less:
Interest and other income	0	0
Property-level operating expenses	64,506	62,085
Office building services costs	727	633
Segment NOI	145,336	140,485
Interest and other income	0	0
Operating Segments | Office Operations | Resident Fees and Services
Revenues:
Other revenues	0	0
Operating Segments | Office Operations | Office Building and Other Services Revenue
Revenues:
Other revenues	2,174	1,775
All Other
Revenues:
Rental income	0	0
Income from loans and investments	24,046	17,126
Interest and other income	4,853	287
Total revenues	29,853	18,156
Less:
Interest and other income	4,853	287
Property-level operating expenses	0	0
Office building services costs	0	0
Segment NOI	25,000	17,869
Interest and other income	4,853	287
All Other | Resident Fees and Services
Revenues:
Other revenues	0	0
All Other | Office Building and Other Services Revenue
Revenues:
Other revenues	$ 954	$ 743