SEGMENT INFORMATION - Income Statement (Details) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2016 USD ($) segment	Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($)
Segment Reporting [Abstract]
Number of reportable business segments | segment	3
Revenues:
Rental income	$ 358,623	$ 325,617
Resident fees and services	463,976	446,914
Medical office building and other services revenue	7,185	10,543
Income from loans and investments	22,386	22,053
Interest and other income	119	471
Total revenues	852,289	805,598
Less:
Interest and other income	119	471
Property-level operating expenses	356,222	340,799
Medical office building services costs	3,451	6,918
Segment NOI	492,497	457,410
Income (loss) from unconsolidated entities	(198)	(251)
Segment profit	492,299	457,159
Interest and other income	119	471
Interest expense	(103,273)	(82,328)
Depreciation and amortization	(236,387)	(216,219)
General, administrative and professional fees	(31,726)	(34,326)
Loss on extinguishment of debt, net	(314)	(21)
Merger-related expenses and deal costs	(1,632)	(30,613)
Other	(4,168)	(4,874)
Income tax benefit	8,421	7,250
Discontinued operations	(489)	17,574
Gain on real estate dispositions	26,184	6,686
Net income	149,034	120,759	$ 419,222
Operating Segments | Triple-Net Leased Properties
Revenues:
Rental income	214,487	188,557
Resident fees and services	0	0
Medical office building and other services revenue	1,199	1,136
Income from loans and investments	0	0
Interest and other income	0	0
Total revenues	215,686	189,693
Less:
Interest and other income	0	0
Property-level operating expenses	0	0
Medical office building services costs	0	0
Segment NOI	215,686	189,693
Income (loss) from unconsolidated entities	(671)	425
Segment profit	215,015	190,118
Interest and other income	0	0
Operating Segments | Senior Living Operations
Revenues:
Rental income	0	0
Resident fees and services	463,976	446,914
Medical office building and other services revenue	0	0
Income from loans and investments	0	0
Interest and other income	0	0
Total revenues	463,976	446,914
Less:
Interest and other income	0	0
Property-level operating expenses	312,541	298,362
Medical office building services costs	0	0
Segment NOI	151,435	148,552
Income (loss) from unconsolidated entities	337	(422)
Segment profit	151,772	148,130
Interest and other income	0	0
Operating Segments | MOB Operations
Revenues:
Rental income	144,136	137,060
Resident fees and services	0	0
Medical office building and other services revenue	4,976	8,858
Income from loans and investments	0	0
Interest and other income	0	0
Total revenues	149,112	145,918
Less:
Interest and other income	0	0
Property-level operating expenses	43,681	42,437
Medical office building services costs	3,451	6,918
Segment NOI	101,980	96,563
Income (loss) from unconsolidated entities	(126)	(25)
Segment profit	101,854	96,538
Interest and other income	0	0
All Other
Revenues:
Rental income	0	0
Resident fees and services	0	0
Medical office building and other services revenue	1,010	549
Income from loans and investments	22,386	22,053
Interest and other income	119	471
Total revenues	23,515	23,073
Less:
Interest and other income	119	471
Property-level operating expenses	0	0
Medical office building services costs	0	0
Segment NOI	23,396	22,602
Income (loss) from unconsolidated entities	262	(229)
Segment profit	23,658	22,373
Interest and other income	$ 119	$ 471