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, 2018	Mar. 31, 2017
Securities in an unrealized gain position:
Fair value (proceeds)	$ 142,133	$ 169,134
Gains realized	8,049	10,738
Securities in an Unrealized Loss Positions [Abstract]
Fair value (proceeds)	56,984	12,452
Losses realized	$ (5,267)	$ (1,257)