SEGMENT INFORMATION - Income Statement (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) segment	Sep. 30, 2015 USD ($)
Segment Reporting [Abstract]
Number of reportable business segments | segment			3
Revenues:
Rental income	$ 368,697	$ 343,783	$ 1,081,526	$ 991,878
Resident fees and services	461,974	454,825	1,390,387	1,356,384
Office building and other services revenue	4,317	10,000	17,006	29,951
Income from loans and investments	31,566	18,924	78,098	66,192
Interest and other income	562	74	792	719
Total revenues	867,116	827,606	2,567,809	2,445,124
Less:
Interest and other income	562	74	792	719
Property-level operating expenses	361,117	347,845	1,069,294	1,031,306
Office building services costs	974	6,416	6,277	19,098
Segment NOI	504,463	473,271	1,491,446	1,394,001
Income (loss) from unconsolidated entities	931	(955)	2,151	(1,197)
Segment profit	505,394	472,316	1,493,597	1,392,804
Interest and other income	562	74	792	719
Interest expense	(105,063)	(97,135)	(312,001)	(263,422)
Depreciation and amortization	(208,387)	(226,332)	(666,735)	(657,262)
General, administrative and professional fees	(31,567)	(32,114)	(95,387)	(100,399)
Loss on extinguishment of debt, net	(383)	(15,331)	(3,165)	(14,897)
Merger-related expenses and deal costs	(16,217)	(62,145)	(25,073)	(105,023)
Other	(2,430)	(4,795)	(8,901)	(13,948)
Income tax benefit	8,537	10,697	28,507	27,736
Income from continuing operations	150,446	45,235	411,634	266,308
Operating Segments | Triple-Net Leased Properties
Revenues:
Rental income	210,424	201,028	635,030	571,591
Resident fees and services	0	0	0	0
Office building and other services revenue	1,246	1,011	3,676	3,286
Income from loans and investments	0	0	0	0
Interest and other income	0	0	0	0
Total revenues	211,670	202,039	638,706	574,877
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	211,670	202,039	638,706	574,877
Income (loss) from unconsolidated entities	584	(1,431)	738	(785)
Segment profit	212,254	200,608	639,444	574,092
Interest and other income	0	0	0	0
Operating Segments | Senior Living Operations
Revenues:
Rental income	0	0	0	0
Resident fees and services	461,974	454,825	1,390,387	1,356,384
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	461,974	454,825	1,390,387	1,356,384
Less:
Interest and other income	0	0	0	0
Property-level operating expenses	312,145	304,540	932,675	902,154
Office building services costs	0	0	0	0
Segment NOI	149,829	150,285	457,712	454,230
Income (loss) from unconsolidated entities	75	433	732	(221)
Segment profit	149,904	150,718	458,444	454,009
Interest and other income	0	0	0	0
Operating Segments | Office Operations
Revenues:
Rental income	158,273	142,755	446,496	420,287
Resident fees and services	0	0	0	0
Office building and other services revenue	2,211	8,459	10,556	25,066
Income from loans and investments	0	0	0	0
Interest and other income	0	0	0	0
Total revenues	160,484	151,214	457,052	445,353
Less:
Interest and other income	0	0	0	0
Property-level operating expenses	48,972	43,305	136,619	129,152
Office building services costs	974	6,416	6,277	19,098
Segment NOI	110,538	101,493	314,156	297,103
Income (loss) from unconsolidated entities	238	108	301	226
Segment profit	110,776	101,601	314,457	297,329
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	860	530	2,774	1,599
Income from loans and investments		18,924	78,098	66,192
Interest and other income		74
Total revenues	32,988	19,528	81,664	68,510
Less:
Interest and other income		74
Property-level operating expenses	0	0	0	0
Office building services costs	0	0	0	0
Segment NOI	32,426	19,454	80,872	67,791
Income (loss) from unconsolidated entities	34	(65)	380	(417)
Segment profit	$ 32,460	19,389	$ 81,252	$ 67,374
Interest and other income		$ 74