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, 2017	Mar. 31, 2016
Securities in an unrealized gain position:
Fair value (proceeds) on securities in an unrealized gain position	$ 169,134	$ 306,231
Gain realized on securities in an unrealized gain position	10,738	9,029
Securities in an unrealized loss position:
Fair value (proceeds) on securities in an unrealized loss position	12,452	53,687
Loss realized on securities in an unrealized loss position	$ (1,257)	$ (3,493)