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		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Securities in an unrealized gain position:
Fair value proceeds	$ 679,514	$ 306,600	$ 1,819,526	$ 914,885
Gains realized	20,155	3,410	34,837	21,596
Securities in an unrealized loss position:
Fair value proceeds	37,488	122,317	375,617	380,493
Losses realized	$ (4,469)	$ (5,428)	$ (12,936)	$ (15,292)