INVESTMENT OPERATIONS - Schedule of Fair Value of Trading Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities - trading	$ 2,625,436	$ 2,457,010
Residential mortgage-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities - trading	209,521	241,836
Commercial mortgage-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities - trading	201,284	188,925
Other asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities - trading	143,361	159,907
U.S. government-related securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities - trading	47,067	59,794
Other government-related securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities - trading	28,775	44,207
States, municipals, and political subdivisions
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities - trading	293,791	286,413
Corporate securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities - trading	1,590,936	1,423,833
Redeemable preferred stocks
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities - trading	12,832	11,277
Fixed Maturities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities - trading	2,527,567	2,416,192
Equity securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities - trading	6,656	9,892
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities - trading	$ 91,213	$ 30,926