INVESTMENT OPERATIONS - Schedule of Fair Value (Proceeds) and Gains/Losses Realized on Securities Sold in an Unrealized Gain/Loss Position (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Securities in an unrealized gain position:
Fair value (proceeds)	$ 1,300,866	$ 879,181	$ 1,198,333
Gains realized	28,095	18,868	42,085
Securities in an unrealized loss position
Fair value (proceeds)	472,371	185,157	85,835
Losses realized	$ (18,183)	$ (8,257)	$ (9,783)