INVESTMENT IN REAL ESTATE PROPERTIES - Net Investment in Direct Financing Lease (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($) Property	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) Property	Jun. 30, 2019 USD ($)	Mar. 31, 2020 USD ($)	Jan. 01, 2020 USD ($)	Dec. 31, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Real Estate [Line Items]
Cumulative effect of ASU adoption	$ 3,361,251	$ 3,260,376	$ 3,361,251	$ 3,260,376	$ 3,395,213		$ 3,488,460	$ 3,050,778	$ 3,254,747
Skilled Nursing Transitional Care Facilities
Real Estate [Line Items]
Net investment in direct financing lease	$ 23,900		$ 23,900
Number of properties in direct financing lease | Property	1		1
Undiscounted rental payments	$ 3,000		$ 3,000
Estimated purchase price	24,700		24,700
Unearned lease income	3,600		3,600
Allowance for credit losses related to direct financing lease	200		200
Lease income	700	$ 700	1,300	$ 1,300
Reduction in the allowance for credit losses related to direct financing lease	21		37
Future minimum lease payments contractually due under the direct financing lease for the remainder of this year	1,100		1,100
Future minimum lease payments contractually due under the direct financing lease due for next year	$ 2,100		$ 2,100
Cumulative Effect in Period of Adoption
Real Estate [Line Items]
Cumulative effect of ASU adoption							$ (167)		$ (32,502)
Cumulative Effect in Period of Adoption | Skilled Nursing Transitional Care Facilities | Topic 326 | Direct Financing Lease
Real Estate [Line Items]
Cumulative effect of ASU adoption						$ (200)