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		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Investments, Debt and Equity Securities [Abstract]
Fair value (proceeds) of securities in an unrealized gain position	$ 121,459	$ 167,272	$ 566,064	$ 990,066
Gains realized on the sale of securities in an unrealized gain position	1,933	3,223	14,967	31,023
Fair value (proceeds) of securities in an unrealized loss position	37,186	7,105	121,450	67,688
Losses realized on the sale of securities in an unrealized loss position	$ (1,243)	$ (1,558)	$ (5,884)	$ (6,871)