SHAREHOLDERS' EQUITY AND PARTNERS' CAPITAL - At-the-Market Equity Program (Details) - USD ($)	12 Months Ended			94 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020	Mar. 01, 2013
Class Of Stock [Line Items]
Common stock offering, maximum aggregate price (up to)					$ 300,000,000
Commission to sales agent (as a percent)					2.00%
Issuance of common stock and restricted common stock (shares)	1,639,236	915,226	727,812
Common stock offering, maximum remaining aggregate price	$ 88,507,000			$ 88,507,000
Common Stock
Class Of Stock [Line Items]
Gross proceeds from issuance of common stock	211,493,000
Net proceeds from issuance of common stock	$ 209,596,000
Issuance of common stock and restricted common stock (shares)				8,419,298
At The Market Stock Sales
Class Of Stock [Line Items]
Proceeds from sale of common stock weighted average price per share (USD per share)	$ 25.12