SEGMENT INFORMATION - Income Statement (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Revenues:
Rental income	$ 401,496	$ 384,809
Income from loans and investments	17,126	31,181
Interest and other income	287	9,634
Total revenues	942,874	943,705
Less:
Interest and other income	287	9,634
Property-level operating expenses	430,504	412,913
Office building services costs	633	115
Segment NOI	511,450	521,043
Loss from unconsolidated entities	(946)	(40,739)
Segment profit	510,504	480,304
Interest and other income	287	9,634
Interest expense	(110,619)	(111,363)
Depreciation and amortization	(235,920)	(233,150)
General, administrative and professional fees	(40,760)	(37,174)
Loss on extinguishment of debt, net	(405)	(10,977)
Merger-related expenses and deal costs	(2,180)	(17,336)
Other	(23)	(3,120)
Gain (Loss) on Disposition of Assets	5,447	48
Income tax benefit	1,257	3,242
Income from continuing operations	127,588	80,108
Discontinued operations	0	(10)
Net income	127,588	80,098
Net income attributable to noncontrolling interests	1,803	1,395
Net income attributable to common stockholders	125,785	78,703
Resident Fees and Services
Revenues:
Other revenues	521,447	514,753
Office Building and Other Services Revenue
Revenues:
Other revenues	2,518	3,328
Operating Segments | Triple-Net Leased Properties
Revenues:
Rental income	200,068	190,641
Income from loans and investments	0	0
Interest and other income	0	0
Total revenues	200,068	191,783
Less:
Interest and other income	0	0
Property-level operating expenses	7,433	0
Office building services costs	0	0
Segment NOI	192,635	191,783
Loss from unconsolidated entities	508	(38,654)
Segment profit	193,143	153,129
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	1,142
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	521,447	514,753
Less:
Interest and other income	0	0
Property-level operating expenses	360,986	352,220
Office building services costs	0	0
Segment NOI	160,461	162,533
Loss from unconsolidated entities	(934)	(641)
Segment profit	159,527	161,892
Interest and other income	0	0
Operating Segments | Senior Living Operations | Resident Fees and Services
Revenues:
Other revenues	521,447	514,753
Operating Segments | Senior Living Operations | Office Building and Other Services Revenue
Revenues:
Other revenues	0	0
Operating Segments | Office Operations
Revenues:
Rental income	201,428	194,168
Income from loans and investments	0	0
Interest and other income	0	0
Total revenues	203,203	195,802
Less:
Interest and other income	0	0
Property-level operating expenses	62,085	60,693
Office building services costs	633	115
Segment NOI	140,485	134,994
Loss from unconsolidated entities	229	(620)
Segment profit	140,714	134,374
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	1,775	1,634
All Other
Revenues:
Rental income	0	0
Income from loans and investments	17,126	31,181
Interest and other income	287	9,634
Total revenues	18,156	41,367
Less:
Interest and other income	287	9,634
Property-level operating expenses	0	0
Office building services costs	0	0
Segment NOI	17,869	31,733
Loss from unconsolidated entities	(749)	(824)
Segment profit	17,120	30,909
Interest and other income	287	9,634
All Other | Resident Fees and Services
Revenues:
Other revenues	0	0
All Other | Office Building and Other Services Revenue
Revenues:
Other revenues	$ 743	$ 552