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, 2019	Dec. 31, 2018	Dec. 31, 2017
Securities in an unrealized gain position:
Fair value proceeds	$ 2,511,764	$ 1,291,826	$ 879,181
Gains realized	61,608	28,034	18,868
Securities in an unrealized loss position:
Fair value proceeds	542,733	472,371	185,157
Losses realized	$ (13,897)	$ (18,183)	$ (8,257)