SEGMENT INFORMATION - Income Statement (Details) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($) segment	Jun. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)
Segment Reporting [Abstract]
Number of reportable business segments | segment			3
Revenues:
Rental income	$ 354,206	$ 322,478	$ 712,829	$ 648,095
Resident fees and services	464,437	454,645	928,413	901,559
Medical office building and other services revenue	5,504	9,408	12,689	19,951
Income from loans and investments	24,146	25,215	46,532	47,268
Interest and other income	111	174	230	645
Total revenues	848,404	811,920	1,700,693	1,617,518
Less:
Interest and other income	111	174	230	645
Property-level operating expenses	351,955	342,662	708,177	683,461
Medical office building services costs	1,852	5,764	5,303	12,682
Segment NOI	494,486	463,320	986,983	920,730
Income (loss) from unconsolidated entities	1,418	9	1,220	(242)
Segment profit	495,904	463,329	988,203	920,488
Interest and other income	111	174	230	645
Interest expense	(103,665)	(83,959)	(206,938)	(166,287)
Depreciation and amortization	(221,961)	(214,711)	(458,348)	(430,930)
General, administrative and professional fees	(32,094)	(33,959)	(63,820)	(68,285)
Gain (loss) on extinguishment of debt, net	(2,468)	455	(2,782)	434
Merger-related expenses and deal costs	(7,224)	(12,265)	(8,856)	(42,878)
Other	(2,303)	(4,279)	(6,471)	(9,153)
Income tax benefit	11,549	9,789	19,970	17,039
Discontinued operations	(148)	18,243	(637)	35,817
Gain on real estate dispositions	5,739	7,469	31,923	14,155
Net income	143,440	150,286	292,474	271,045	$ 419,222
Operating Segments | Triple-Net Leased Properties
Revenues:
Rental income	210,119	182,006	424,606	370,563
Resident fees and services	0	0	0	0
Medical office building and other services revenue	1,231	1,139	2,430	2,275
Income from loans and investments	0	0	0	0
Interest and other income	0	0	0	0
Total revenues	211,350	183,145	427,036	372,838
Less:
Interest and other income	0	0	0	0
Property-level operating expenses	0	0	0	0
Medical office building services costs	0	0	0	0
Segment NOI	211,350	183,145	427,036	372,838
Income (loss) from unconsolidated entities	825	221	154	645
Segment profit	212,175	183,366	427,190	373,483
Interest and other income	0	0	0	0
Operating Segments | Senior Living Operations
Revenues:
Rental income	0	0	0	0
Resident fees and services	464,437	454,645
Medical office building and other services revenue	0	0	0	0
Income from loans and investments	0	0	0	0
Interest and other income	0	0	0	0
Total revenues	464,437	454,645	928,413	901,559
Less:
Interest and other income	0	0	0	0
Property-level operating expenses	307,989	299,252	620,530	597,614
Medical office building services costs	0	0	0	0
Segment NOI	156,448	155,393	307,883	303,945
Income (loss) from unconsolidated entities	336	(240)	673	(662)
Segment profit	156,784	155,153	308,556	303,283
Interest and other income	0	0	0	0
Operating Segments | MOB Operations
Revenues:
Rental income	144,087	140,472	288,223	277,532
Resident fees and services	0	0	0	0
Medical office building and other services revenue	3,369	7,749	8,345	16,607
Income from loans and investments	0	0	0	0
Interest and other income	0	0	0	0
Total revenues	147,456	148,221	296,568	294,139
Less:
Interest and other income	0	0	0	0
Property-level operating expenses	43,966	43,410	87,647	85,847
Medical office building services costs	1,852	5,764	5,303	12,682
Segment NOI	101,638	99,047	203,618	195,610
Income (loss) from unconsolidated entities	173	143	47	119
Segment profit	101,811	99,190	203,665	195,729
Interest and other income	0	0	0	0
All Other
Revenues:
Rental income	0	0	0	0
Resident fees and services	0	0	0	0
Medical office building and other services revenue	904	520	1,914	1,069
Income from loans and investments	24,146	25,215
Interest and other income	111	174	230	645
Total revenues	25,161	25,909	48,676	48,982
Less:
Interest and other income	111	174	230	645
Property-level operating expenses	0	0	0	0
Medical office building services costs	0	0	0	0
Segment NOI	25,050	25,735	48,446	48,337
Income (loss) from unconsolidated entities	84	(115)	346	(344)
Segment profit	25,134	25,620	48,792	47,993
Interest and other income	$ 111	$ 174	$ 230	$ 645