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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Securities in an unrealized gain position:
Fair value (proceeds)	$ 461,113	$ 275,470	$ 603,246	$ 444,604
Gains realized	10,082	2,297	18,131	13,035
Securities in an unrealized loss position:
Fair value (proceeds)	201,191	71,813	258,175	84,265
Losses realized	$ (4,598)	$ (3,384)	$ (9,865)	$ (4,641)