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)	$ 491,121	$ 466,153	$ 1,140,012	$ 608,286
Gains realized	6,812	10,137	14,682	18,187
Securities in an unrealized loss position:
Fair value (proceeds)	160,125	201,191	338,129	258,175
Losses realized	$ (5,714)	$ (4,598)	$ (8,467)	$ (9,865)