INVESTMENT OPERATIONS - Schedule of Fair Value of Trading Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities—trading	$ 2,457,010	$ 2,725,694
Fixed maturities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities—trading	2,416,192	2,664,189
Residential mortgage-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities—trading	241,836	259,694
Commercial mortgage-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities—trading	188,925	146,804
Other asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities—trading	159,907	138,097
U.S. government-related securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities—trading	59,794	27,234
Other government-related securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities—trading	44,207	63,925
States, municipals, and political subdivisions
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities—trading	286,413	326,925
Corporate securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities—trading	1,423,833	1,698,183
Redeemable preferred stock
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities—trading	11,277	3,327
Equity securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities—trading	9,892	5,244
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities—trading	$ 30,926	$ 56,261