INVESTMENT OPERATIONS INVESTMENT OPERATIONS - Schedule of Fair Value of Trading Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	$ 2,725,694	$ 2,699,591
Fixed maturities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	2,664,189	2,639,860
Residential mortgage-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	259,694	255,027
Commercial mortgage-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	146,804	149,683
Other asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	138,097	200,084
U.S. government-related securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	27,234	26,961
Other government-related securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	63,925	63,012
States, municipals, and political subdivisions
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	326,925	316,519
Corporate securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	1,698,183	1,624,589
Redeemable preferred stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	3,327	3,985
Equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	5,244	7,083
Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	$ 56,261	$ 52,648