INVESTMENT OPERATIONS - Schedule of Fair Value (Proceeds) and Gains/Losses Realized on Securities Sold in an Unrealized Gain/Loss Position (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Securities in an unrealized gain position:
Fair value proceeds	$ 431,876	$ 484,587	$ 936,213	$ 1,133,478
Gains realized	6,471	6,776	46,439	14,646
Securities in an unrealized loss position:
Fair value proceeds	24,814	159,626	24,834	330,928
Losses realized	$ (4,024)	$ (5,714)	$ (4,822)	$ (8,447)