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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Securities in an unrealized gain position:
Fair value (proceeds)	$ 484,587	$ 461,113	$ 1,133,478	$ 603,246
Gains realized	6,776	10,082	14,646	18,131
Securities in an unrealized loss position:
Fair value (proceeds)	159,626	201,191	330,928	281,175
Losses realized	$ (5,714)	$ (4,598)	$ (8,447)	$ (9,865)