INVESTMENT OPERATIONS - Schedule of Fair Value of Trading Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Trading Securities [Line Items]
Trading securities	$ 2,550,330	$ 2,725,694
Residential mortgage-backed securities
Trading Securities [Line Items]
Trading securities	259,755	259,694
Commercial mortgage-backed securities
Trading Securities [Line Items]
Trading securities	141,469	146,804
Other asset-backed securities
Trading Securities [Line Items]
Trading securities	114,507	138,097
U.S. government-related securities
Trading Securities [Line Items]
Trading securities	36,575	27,234
Other government-related securities
Trading Securities [Line Items]
Trading securities	40,274	63,925
States, municipals, and political subdivisions
Trading Securities [Line Items]
Trading securities	314,722	326,925
Corporate securities
Trading Securities [Line Items]
Trading securities	1,585,290	1,698,183
Redeemable preferred stock
Trading Securities [Line Items]
Trading securities	3,289	3,327
Fixed maturities
Trading Securities [Line Items]
Trading securities	2,495,881	2,664,189
Equity securities
Trading Securities [Line Items]
Trading securities	5,332	5,244
Short-term investments
Trading Securities [Line Items]
Trading securities	$ 49,117	$ 56,261