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		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Securities in an unrealized gain position:
Fair value proceeds	$ 415,440	$ 679,514	$ 1,351,653	$ 1,812,992
Gains realized	2,750	20,155	49,189	34,801
Securities in an unrealized loss position:
Fair value proceeds	8,228	37,488	33,062	368,416
Losses realized	$ (69)	$ (4,469)	$ (4,891)	$ (12,916)