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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Securities in an unrealized gain position:
Fair value (proceeds) on securities in an unrealized gain position	$ 275,470	$ 513,544	$ 444,604	$ 819,775
Gain realized on securities in an unrealized gain position	2,297	18,752	13,035	27,781
Securities in an unrealized loss position:
Fair value (proceeds) on securities in an unrealized loss position	71,813	6,895	84,265	60,582
Loss realized on securities in an unrealized loss position	$ (3,384)	$ (1,820)	$ (4,641)	$ (5,313)