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
	Mar. 31, 2019	Mar. 31, 2018
Securities in an unrealized gain position:
Fair value (proceeds)	$ 648,891	$ 142,133
Gains realized	7,870	8,049
Securities in an unrealized loss position:
Fair value (proceeds)	178,004	56,984
Losses realized	$ (2,753)	$ (5,267)