SEGMENT INFORMATION - Income Statement (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Revenues:
Rental income	$ 408,322	$ 384,028	$ 1,208,388	$ 1,129,099
Income from loans and investments	30,164	18,108	66,819	105,706
Interest and other income	620	12,554	10,109	24,535
Total revenues	983,155	936,538	2,876,746	2,822,547
Less:
Interest and other income	620	12,554	10,109	24,535
Property-level operating expenses	461,493	428,389	1,327,898	1,262,715
Office building services costs	627	431	1,775	1,080
Segment NOI	520,415	495,164	1,536,964	1,534,217
Interest and other income	620	12,554	10,109	24,535
Interest expense	(113,967)	(107,581)	(334,955)	(331,973)
Depreciation and amortization	(234,603)	(218,579)	(696,710)	(675,363)
General, administrative and professional fees	(40,530)	(39,677)	(124,369)	(113,507)
Loss on extinguishment of debt, net	37,434	39,527	41,861	50,411
Merger-related expenses and deal costs	(4,304)	(4,458)	(11,084)	(26,288)
Other	(2,164)	(1,244)	9,294	(7,891)
Income (loss) from unconsolidated entities	854	(716)	(2,621)	(47,826)
Gain on real estate dispositions	36	18	24,633	35,893
Income tax (expense) benefit	(2,005)	7,327	57,004	11,303
Income from continuing operations	86,918	103,281	426,404	352,689
Discontinued operations	0	0	0	(10)
Net income	86,918	103,281	426,404	352,679
Net income attributable to noncontrolling interests	1,659	1,309	4,831	5,485
Net income attributable to common stockholders	85,259	101,972	421,573	347,194
Resident Fees and Services
Revenues:
Other revenues	541,090	518,560	1,583,262	1,552,302
Office Building and Other Services Revenue
Revenues:
Other revenues	2,959	3,288	8,168	10,905
Operating Segments | Triple-Net Leased Properties
Revenues:
Rental income	193,383	190,117	589,833	548,628
Income from loans and investments	0	0	0	0
Interest and other income	0	0	0	0
Total revenues	193,383	190,319	589,833	551,150
Less:
Interest and other income	0	0	0	0
Property-level operating expenses	6,338	0	20,092	0
Office building services costs	0	0	0	0
Segment NOI	187,045	190,319	569,741	551,150
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	202	0	2,522
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	541,090	518,560	1,583,262	1,552,302
Less:
Interest and other income	0	0	0	0
Property-level operating expenses	388,011	366,721	1,115,834	1,080,053
Office building services costs	0	0	0	0
Segment NOI	153,079	151,839	467,428	472,249
Interest and other income	0	0	0	0
Operating Segments | Senior Living Operations | Resident Fees and Services
Revenues:
Other revenues	541,090	518,560	1,583,262	1,552,302
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	214,939	193,911	618,555	580,471
Income from loans and investments	0	0	0	0
Interest and other income	0	0	0	0
Total revenues	216,998	196,086	624,240	586,256
Less:
Interest and other income	0	0	0	0
Property-level operating expenses	67,144	61,668	191,972	182,662
Office building services costs	627	431	1,775	1,080
Segment NOI	149,227	133,987	430,493	402,514
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	2,059	2,175	5,685	5,785
All Other
Revenues:
Rental income	0	0	0	0
Income from loans and investments	30,164	18,108	66,819	105,706
Interest and other income	620	12,554	10,109	24,535
Total revenues	31,684	31,573	79,411	132,839
Less:
Interest and other income	620	12,554	10,109	24,535
Property-level operating expenses	0	0	0	0
Office building services costs	0	0	0	0
Segment NOI	31,064	19,019	69,302	108,304
Interest and other income	620	12,554	10,109	24,535
All Other | Resident Fees and Services
Revenues:
Other revenues	0	0	0	0
All Other | Office Building and Other Services Revenue
Revenues:
Other revenues	$ 900	$ 911	$ 2,483	$ 2,598