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
Investments, Debt and Equity Securities [Abstract]
Fair value (proceeds) of securities in an unrealized gain position	$ 275,470	$ 513,544	$ 444,604	$ 822,793
Gain realized on the sale of securities in an unrealized gain position	2,297	18,752	13,035	27,800
Fair value (proceeds) of securities in an unrealized loss position	71,813	6,895	84,265	60,582
Loss realized on the sale of securities in an unrealized loss position	$ (3,384)	$ (1,820)	$ (4,641)	$ (5,313)