INVESTMENT OPERATIONS - Schedule of Fair Value of Trading Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt and Equity Securities, FV-NI [Line Items]
Trading securities	$ 2,531,422	$ 2,725,694
Residential mortgage-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Trading securities	254,037	259,694
Commercial mortgage-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Trading securities	135,403	146,804
Other asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Trading securities	111,770	138,097
U.S. government-related securities
Debt and Equity Securities, FV-NI [Line Items]
Trading securities	58,081	27,234
Other government-related securities
Debt and Equity Securities, FV-NI [Line Items]
Trading securities	43,030	63,925
States, municipals, and political subdivisions
Debt and Equity Securities, FV-NI [Line Items]
Trading securities	296,618	326,925
Corporate securities
Debt and Equity Securities, FV-NI [Line Items]
Trading securities	1,569,652	1,698,183
Redeemable preferred stock
Debt and Equity Securities, FV-NI [Line Items]
Trading securities	3,165	3,327
Fixed maturities
Debt and Equity Securities, FV-NI [Line Items]
Trading securities	2,471,756	2,664,189
Equity securities
Debt and Equity Securities, FV-NI [Line Items]
Trading securities	3,801	5,244
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Trading securities	$ 55,865	$ 56,261