SEGMENT INFORMATION - Income Statement (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenues:
Rental income	$ 384,028	$ 401,876	$ 1,129,099	$ 1,196,596
Income from loans and investments	18,108	32,985	105,706	85,499
Interest and other income	12,554	171	24,535	854
Total revenues	936,538	899,928	2,822,547	2,678,861
Less:
Interest and other income	12,554	171	24,535	854
Property-level operating expenses	428,389	376,207	1,262,715	1,111,024
Office building services costs	431	418	1,080	1,708
Segment NOI	495,164	523,132	1,534,217	1,565,275
(Loss) income from unconsolidated entities	(716)	750	(47,826)	3,794
Segment profit	494,448	523,882	1,486,391	1,569,069
Interest and other income	12,554	171	24,535	854
Interest expense	(107,581)	(113,869)	(331,973)	(336,245)
Depreciation and amortization	(218,579)	(213,407)	(675,363)	(655,298)
General, administrative and professional fees	(39,677)	(33,317)	(113,507)	(100,560)
Loss on extinguishment of debt, net	(39,527)	(511)	(50,411)	(856)
Merger-related expenses and deal costs	(4,458)	(804)	(26,288)	(8,903)
Other	(1,244)	(13,030)	(7,891)	(16,066)
Income tax benefit	7,327	7,815	11,303	13,119
Income from continuing operations	103,263	156,930	316,796	465,114
Resident Fees and Services
Revenues:
Other revenues	518,560	461,700	1,552,302	1,386,131
Office Building and Other Services Revenue
Revenues:
Other revenues	3,288	3,196	10,905	9,781
Operating Segments | Triple-Net Leased Properties
Revenues:
Rental income	190,117	212,370	548,628	634,955
Income from loans and investments	0	0	0	0
Interest and other income	0	0	0	0
Total revenues	190,319	213,495	551,150	638,410
Less:
Interest and other income	0	0	0	0
Property-level operating expenses	0	0	0	0
Office building services costs	0	0	0	0
Segment NOI	190,319	213,495	551,150	638,410
(Loss) income from unconsolidated entities	1,320	1,122	(42,029)	4,768
Segment profit	191,639	214,617	509,121	643,178
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	202	1,125	2,522	3,455
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	518,560	461,700	1,552,302	1,386,131
Less:
Interest and other income	0	0	0	0
Property-level operating expenses	366,721	315,598	1,080,053	936,296
Office building services costs	0	0	0	0
Segment NOI	151,839	146,102	472,249	449,835
(Loss) income from unconsolidated entities	(1,314)	300	(3,668)	(157)
Segment profit	150,525	146,402	468,581	449,678
Interest and other income	0	0	0	0
Operating Segments | Senior Living Operations | Resident Fees and Services
Revenues:
Other revenues	518,560	461,700	1,552,302	1,386,131
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	193,911	189,506	580,471	561,641
Income from loans and investments	0	0	0	0
Interest and other income	0	0	0	0
Total revenues	196,086	191,074	586,256	566,988
Less:
Interest and other income	0	0	0	0
Property-level operating expenses	61,668	60,609	182,662	174,728
Office building services costs	431	418	1,080	1,708
Segment NOI	133,987	130,047	402,514	390,552
(Loss) income from unconsolidated entities	56	(348)	321	284
Segment profit	134,043	129,699	402,835	390,836
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	2,175	1,568	5,785	5,347
All Other
Revenues:
Rental income	0	0	0	0
Income from loans and investments	18,108	32,985	105,706	85,499
Interest and other income	12,554	171	24,535	854
Total revenues	31,573	33,659	132,839	87,332
Less:
Interest and other income	12,554	171	24,535	854
Property-level operating expenses	0	0	0	0
Office building services costs	0	0	0	0
Segment NOI	19,019	33,488	108,304	86,478
(Loss) income from unconsolidated entities	(778)	(324)	(2,450)	(1,101)
Segment profit	18,241	33,164	105,854	85,377
Interest and other income	12,554	171	24,535	854
All Other | Resident Fees and Services
Revenues:
Other revenues	0	0	0	0
All Other | Office Building and Other Services Revenue
Revenues:
Other revenues	$ 911	$ 503	$ 2,598	$ 979