ACQUISITIONS OF REAL ESTATE PROPERTY - Fair Value of the Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
2016 Acquisitions
Business Acquisition [Line Items]
Land and improvements	$ 49,645
Buildings and improvements	1,335,156
Acquired lease intangibles	203,337
Other assets	98,137
Total assets acquired	1,686,275
Notes payable and other debt	47,641
Intangible liabilities	108,132
Other liabilities	60,613
Total liabilities assumed	216,386
Noncontrolling interest assumed	22,225
Net assets acquired	1,447,664
Cash acquired	19,119
Total cash used	1,428,545
2016 Acquisitions | Triple-Net Leased Properties
Business Acquisition [Line Items]
Land and improvements	1,279
Buildings and improvements	12,558
Acquired lease intangibles	163
Other assets	0
Total assets acquired	14,000
Notes payable and other debt	0
Intangible liabilities	0
Other liabilities	380
Total liabilities assumed	380
Noncontrolling interest assumed	0
Net assets acquired	13,620
Cash acquired	0
Total cash used	13,620
2016 Acquisitions | Office Operations
Business Acquisition [Line Items]
Land and improvements	48,366
Buildings and improvements	1,322,598
Acquired lease intangibles	203,174
Other assets	98,137
Total assets acquired	1,672,275
Notes payable and other debt	47,641
Intangible liabilities	108,132
Other liabilities	60,233
Total liabilities assumed	216,006
Noncontrolling interest assumed	22,225
Net assets acquired	1,434,044
Cash acquired	19,119
Total cash used	$ 1,414,925
2015 Acquisitions
Business Acquisition [Line Items]
Land and improvements		$ 434,586
Buildings and improvements		3,643,547
Acquired lease intangibles		437,769
Other assets		850,027
Total assets acquired		5,365,929
Notes payable and other debt		177,857
Other liabilities		138,066
Total liabilities assumed		315,923
Noncontrolling interest assumed		88,085
Net assets acquired		5,050,006
Cash acquired		59,584
Equity issued		2,216,355
Total cash used		2,685,982
2015 Acquisitions | Triple-Net Leased Properties
Business Acquisition [Line Items]
Land and improvements		190,566
Buildings and improvements		1,726,064
Acquired lease intangibles		169,362
Other assets		174,093
Total assets acquired		2,260,085
Notes payable and other debt		0
Other liabilities		45,924
Total liabilities assumed		45,924
Net assets acquired		2,214,161
2015 Acquisitions | Senior Living Operations
Business Acquisition [Line Items]
Land and improvements		70,713
Buildings and improvements		703,080
Acquired lease intangibles		83,867
Other assets		272,888
Total assets acquired		1,130,548
Notes payable and other debt		77,940
Other liabilities		45,408
Total liabilities assumed		123,348
Net assets acquired		1,007,200
2015 Acquisitions | Office Operations
Business Acquisition [Line Items]
Land and improvements		173,307
Buildings and improvements		1,214,403
Acquired lease intangibles		184,540
Other assets		403,046
Total assets acquired		1,975,296
Notes payable and other debt		99,917
Other liabilities		46,734
Total liabilities assumed		146,651
Net assets acquired		$ 1,828,645