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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Securities in an unrealized gain position:
Fair value (proceeds)	$ 306,600	$ 121,459	$ 909,846	$ 566,064
Gains realized	3,410	1,933	21,540	14,967
Securities in an unrealized loss position:
Fair value (proceeds)	122,317	37,186	380,493	121,450
Losses realized	$ (5,428)	$ (1,243)	$ (15,292)	$ (5,884)