DEBT AND OTHER OBLIGATIONS - Secured Financing Transactions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Secured borrowings, percent of initial collateral required	102.00%
Securities lending transactions	$ 56,616	$ 62,840
Securities loaned, fair value of collateral	58,700	65,500
Repurchase Program Borrowings
Debt Instrument [Line Items]
Fair value of securities pledged under the repurchase program	452,100	282,200
Secured financing liabilities	$ 437,000	$ 270,000
Reverse repurchase liability average interest rate (percent)	0.15%	1.63%
Reverse repurchase maximum balance outstanding	$ 824,700	$ 900,000
Reverse repurchase average daily balance	$ 143,200	$ 212,200
Reverse repurchase average daily balance at average interest rate (percent)	0.33%	2.14%
Repurchase Program Borrowings | Maximum
Debt Instrument [Line Items]
Term of financing agreement	90 days