FAIR VALUE - Financial Instruments Other than Debt Securities (Details) - Fair Value, Inputs, Level 2 - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Carrying Amount [Member]
Financial liabilities [Abstract]
Unsecured borrowings	$ 98,544	$ 98,493
Secured borrowings with initial terms of greater than 120 days	500,000	500,000
Fair Value [Member]
Financial liabilities [Abstract]
Unsecured borrowings	74,200	59,900
Secured borrowings with initial terms of greater than 120 days	500,100	500,100
Interest Rate Swap Agreements [Member] | Carrying Amount [Member] | Unsecured Borrowings [Member]
Financial liabilities [Abstract]
Interest rate swap agreements	33,335	41,484
Interest Rate Swap Agreements [Member] | Fair Value [Member] | Unsecured Borrowings [Member]
Financial liabilities [Abstract]
Interest rate swap agreements	$ 33,335	$ 41,484