INVESTMENT OPERATIONS - Schedule of Fair Value (Proceeds) and Gains/Losses Realized on Securities Sold in an Unrealized Gail/Loss Position (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Securities in an unrealized gain position:
Fair value proceeds	$ 1,713,162	$ 2,511,764	$ 1,291,826
Gains realized	50,247	61,608	28,034
Securities in an unrealized loss position:
Fair value proceeds	33,816	542,733	472,371
Losses realized	$ (4,923)	$ (13,897)	$ (18,183)