ACQUISITIONS OF REAL ESTATE PROPERTY - Narrative (Details) $ / shares in Units, $ in Thousands, shares in Millions			1 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended
	Jul. 05, 2016 USD ($) property	Aug. 04, 2015 USD ($)	Aug. 31, 2015 property	Jan. 31, 2015 USD ($) property $ / shares shares	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) property	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($) property
Business Acquisition [Line Items]
Investment in unconsolidated operating entity							$ 0	$ 26,282
Total net real estate property					$ 20,601,402		20,601,402		$ 19,625,220
Total revenues					867,116	$ 827,606	2,567,809	$ 2,445,124
Goodwill					1,043,075		1,043,075		$ 1,047,497
Limited Partnership Units
Business Acquisition [Line Items]
Equity interest issued, number of shares | shares				1.1
Wexford Acquisition
Business Acquisition [Line Items]
Total cash used	$ 1,500,000
Wexford Acquisition | Operating Property
Business Acquisition [Line Items]
Number of real estate properties acquired | property	23
Wexford Acquisition | Development Assets
Business Acquisition [Line Items]
Number of real estate properties acquired | property	2
Wexford Acquisition | Future Development Sites
Business Acquisition [Line Items]
Number of real estate properties acquired | property	9
Other 2016 Acquisitions
Business Acquisition [Line Items]
Investment in unconsolidated operating entity							42,000
Other 2016 Acquisitions | Triple-Net Leased Properties
Business Acquisition [Line Items]
Number of real estate properties acquired | property									1
Other 2016 Acquisitions | Unconsolidated Properties | Office Operations
Business Acquisition [Line Items]
Number of real estate properties acquired | property									2
2016 Acquisitions
Business Acquisition [Line Items]
Total cash used							1,428,545
Acquisition costs					14,000		18,600
Assumed mortgage debt					47,641		47,641
2016 Acquisitions | Triple-Net Leased Properties
Business Acquisition [Line Items]
Total cash used							13,620
Assumed mortgage debt					0		0
2016 Acquisitions | Office Operations
Business Acquisition [Line Items]
Total cash used							1,414,925
Assumed mortgage debt					47,641		$ 47,641
2016 Acquisitions | Development Assets
Business Acquisition [Line Items]
Number of real estate properties completed | property							3
Total net real estate property					31,600		$ 31,600
2016 Acquisitions | Expansion of Senior Housing
Business Acquisition [Line Items]
Number of real estate properties completed | property							2
2015 Acquisitions
Business Acquisition [Line Items]
Total cash used									$ 2,685,982
Number of real estate properties acquired | property				152
Assumed mortgage debt									177,857
Equity issued									2,216,355
Goodwill									746,900
2015 Acquisitions | Triple-Net Leased Properties
Business Acquisition [Line Items]
Assumed mortgage debt									0
Goodwill									133,600
2015 Acquisitions | Senior Living Operations
Business Acquisition [Line Items]
Assumed mortgage debt									77,940
Goodwill									219,100
2015 Acquisitions | Office Operations
Business Acquisition [Line Items]
Assumed mortgage debt									99,917
Goodwill									394,200
American Realty Capital Healthcare Trust
Business Acquisition [Line Items]
Total cash used				$ 11,000
Number of real estate properties acquired | property				20
Consideration transferred, stock conversion ratio, per acquiree share				0.1688
Consideration transferred, price per acquiree share | $ / shares				$ 11.33
Assumed mortgage debt				$ 167,000
Repayments of debt				$ 730,000
American Realty Capital Healthcare Trust | Skilled Nursing Facilities
Business Acquisition [Line Items]
Number of real estate properties acquired | property			10
American Realty Capital Healthcare Trust | Common Stock Par Value
Business Acquisition [Line Items]
Equity interest issued, number of shares | shares				28.4
Ardent
Business Acquisition [Line Items]
Investment in unconsolidated operating entity		$ 26,300
Business combination assets acquired, property plant and equipment		$ 1,300,000
Ownership interests accounted for under the equity method		9.90%
Other 2015 Acquisitions
Business Acquisition [Line Items]
Other investments									$ 612,000
Other 2015 Acquisitions | Triple-Net Leased Properties
Business Acquisition [Line Items]
Number of real estate properties acquired | property									11
Other 2015 Acquisitions | Office Operations
Business Acquisition [Line Items]
Number of real estate properties acquired | property									11
Other 2015 Acquisitions | Skilled Nursing Facilities
Business Acquisition [Line Items]
Number of real estate properties acquired | property									12
Other 2015 Acquisitions | Unconsolidated Properties | Office Operations
Business Acquisition [Line Items]
Number of real estate properties acquired | property									8
2015 Development Projects | Triple-Net Leased Properties
Business Acquisition [Line Items]
Number of real estate properties completed | property									1
Total net real estate property									$ 9,300
Aggregate Revenue | 2016 Acquisitions
Business Acquisition [Line Items]
Total revenues					14,000		$ 14,700
Net Operating Income | 2016 Acquisitions
Business Acquisition [Line Items]
Total revenues					$ 9,600		$ 10,200