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
Investments, Debt and Equity Securities [Abstract]
Fair value (proceeds) of securities in an unrealized gain position	$ 169,134	$ 309,249
Gain realized on the sale of securities in an unrealized gain position	10,738	9,048
Fair value (proceeds) of securities in an unrealized loss position	12,452	53,687
Loss realized on the sale of securities in an unrealized loss position	$ (1,257)	$ (3,493)