SEGMENT INFORMATION - Income Statement (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) segment	Jun. 30, 2016 USD ($)
Segment Reporting [Abstract]
Number of reportable business segments | segment			3
Revenues:
Rental income	$ 399,498	$ 354,206	$ 794,720	$ 712,829
Resident fees and services	460,243	464,437	924,431	928,413
Office building and other services revenue	3,179	5,504	6,585	12,689
Income from loans and investments	32,368	24,146	52,514	46,532
Interest and other income	202	111	683	230
Total revenues	895,490	848,404	1,778,933	1,700,693
Less:
Interest and other income	202	111	683	230
Property-level operating expenses	365,830	351,955	734,817	708,177
Office building services costs	552	1,852	1,290	5,303
Segment NOI	528,906	494,486	1,042,143	986,983
Income (loss) from unconsolidated entities	(106)	1,418	3,044	1,220
Segment profit	528,800	495,904	1,045,187	988,203
Interest and other income	202	111	683	230
Interest expense	(113,572)	(103,665)	(222,376)	(206,938)
Depreciation and amortization	(224,108)	(221,961)	(441,891)	(458,348)
General, administrative and professional fees	(33,282)	(32,094)	(67,243)	(63,820)
Loss on extinguishment of debt, net	(36)	(2,468)	(345)	(2,782)
Merger-related expenses and deal costs	(6,043)	(7,224)	(8,099)	(8,856)
Other	(1,848)	(2,303)	(3,036)	(6,471)
Income tax benefit	2,159	11,549	5,304	19,970
Income from continuing operations	152,272	137,849	308,184	261,188
Operating Segments | Triple-Net Leased Properties
Revenues:
Rental income	213,258	210,119	422,585	424,606
Resident fees and services	0	0	0	0
Office building and other services revenue	1,125	1,231	2,330	2,430
Income from loans and investments	0	0	0	0
Interest and other income	0	0	0	0
Total revenues	214,383	211,350	424,915	427,036
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	214,383	211,350	424,915	427,036
Income (loss) from unconsolidated entities	377	825	3,646	154
Segment profit	214,760	212,175	428,561	427,190
Interest and other income	0	0	0	0
Operating Segments | Senior Living Operations
Revenues:
Rental income	0	0	0	0
Resident fees and services	460,243	464,437	924,431	928,413
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	460,243	464,437	924,431	928,413
Less:
Interest and other income	0	0	0	0
Property-level operating expenses	308,625	307,989	620,698	620,530
Office building services costs	0	0	0	0
Segment NOI	151,618	156,448	303,733	307,883
Income (loss) from unconsolidated entities	(381)	336	(457)	673
Segment profit	151,237	156,784	303,276	308,556
Interest and other income	0	0	0	0
Operating Segments | Office Operations
Revenues:
Rental income	186,240	144,087	372,135	288,223
Resident fees and services	0	0	0	0
Office building and other services revenue	1,848	3,369	3,779	8,345
Income from loans and investments	0	0	0	0
Interest and other income	0	0	0	0
Total revenues	188,088	147,456	375,914	296,568
Less:
Interest and other income	0	0	0	0
Property-level operating expenses	57,205	43,966	114,119	87,647
Office building services costs	552	1,852	1,290	5,303
Segment NOI	130,331	101,638	260,505	203,618
Income (loss) from unconsolidated entities	298	173	633	47
Segment profit	130,629	101,811	261,138	203,665
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
Office building and other services revenue	206	904	476	1,914
Income from loans and investments	32,368	24,146	52,514	46,532
Interest and other income	202	111	683	230
Total revenues	32,776	25,161	53,673	48,676
Less:
Interest and other income	202	111	683	230
Property-level operating expenses	0	0	0	0
Office building services costs	0	0	0	0
Segment NOI	32,574	25,050	52,990	48,446
Income (loss) from unconsolidated entities	(400)	84	(778)	346
Segment profit	32,174	25,134	52,212	48,792
Interest and other income	$ 202	$ 111	$ 683	$ 230