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
	Mar. 31, 2020	Mar. 31, 2019
Securities in an unrealized gain position:
Fair value proceeds	$ 504,337	$ 648,891
Gains realized	39,968	7,870
Securities in an unrealized loss position:
Fair value proceeds	15	171,302
Losses realized	$ (798)	$ (2,733)