INVESTMENT OPERATIONS - Schedule of Fair Value of Trading Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Trading Securities [Line Items]
Trading securities	$ 2,662,999	$ 2,725,694
Residential mortgage-backed securities
Trading Securities [Line Items]
Trading securities	265,547	259,694
Commercial mortgage-backed securities
Trading Securities [Line Items]
Trading securities	146,633	146,804
Other asset-backed securities
Trading Securities [Line Items]
Trading securities	129,714	138,097
U.S. government-related securities
Trading Securities [Line Items]
Trading securities	37,575	27,234
Other government-related securities
Trading Securities [Line Items]
Trading securities	40,368	63,925
States, municipals, and political subdivisions
Trading Securities [Line Items]
Trading securities	318,142	326,925
Corporate securities
Trading Securities [Line Items]
Trading securities	1,645,899	1,698,183
Preferred stock
Trading Securities [Line Items]
Trading securities	3,264	3,327
Fixed maturities
Trading Securities [Line Items]
Trading securities	2,587,142	2,664,189
Equity securities
Trading Securities [Line Items]
Trading securities	5,366	5,244
Short-term investments
Trading Securities [Line Items]
Trading securities	$ 70,491	$ 56,261