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, 2015	Dec. 31, 2014
2015 Acquisitions
Business Acquisition [Line Items]
Land and improvements	$ 432,929
Buildings and improvements	3,555,359
Acquired lease intangibles	431,394
Other assets	844,764
Total assets acquired	5,264,446
Notes payable and other debt	177,857
Other liabilities	136,042
Total liabilities assumed	313,899
Net assets acquired	4,950,547
Redeemable OP unitholder interests assumed	87,245
Cash acquired	54,778
Equity issued	2,216,355
Total cash used	2,592,169
2015 Acquisitions | Triple-Net Leased Properties
Business Acquisition [Line Items]
Land and improvements	190,566
Buildings and improvements	1,726,553
Acquired lease intangibles	169,362
Other assets	173,144
Total assets acquired	2,259,625
Notes payable and other debt	0
Other liabilities	43,841
Total liabilities assumed	43,841
Net assets acquired	2,215,784
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	273,523
Total assets acquired	1,131,183
Notes payable and other debt	77,940
Other liabilities	43,412
Total liabilities assumed	121,352
Net assets acquired	1,009,831
2015 Acquisitions | MOB Operations
Business Acquisition [Line Items]
Land and improvements	171,650
Buildings and improvements	1,125,726
Acquired lease intangibles	178,165
Other assets	398,097
Total assets acquired	1,873,638
Notes payable and other debt	99,917
Other liabilities	48,789
Total liabilities assumed	148,706
Net assets acquired	$ 1,724,932
2014 Acquisitions
Business Acquisition [Line Items]
Land and improvements		$ 145,867
Buildings and improvements		1,628,479
Acquired lease intangibles		65,335
Other assets		12,620
Total assets acquired		1,852,301
Notes payable and other debt		241,077
Other liabilities		133,569
Total liabilities assumed		374,646
Net assets acquired		1,477,655
Cash acquired		8,931
Total cash used		1,468,724
2014 Acquisitions | Triple-Net Leased Properties
Business Acquisition [Line Items]
Land and improvements		45,586
Buildings and improvements		546,849
Acquired lease intangibles		28,883
Other assets		227
Total assets acquired		621,545
Notes payable and other debt		12,927
Other liabilities		8,609
Total liabilities assumed		21,536
Net assets acquired		600,009
Cash acquired		227
Total cash used		599,782
2014 Acquisitions | Senior Living Operations
Business Acquisition [Line Items]
Land and improvements		100,281
Buildings and improvements		1,081,630
Acquired lease intangibles		36,452
Other assets		12,393
Total assets acquired		1,230,756
Notes payable and other debt		228,150
Other liabilities		124,960
Total liabilities assumed		353,110
Net assets acquired		877,646
Cash acquired		8,704
Total cash used		$ 868,942