INVESTMENT OPERATIONS - Schedule of Fair Value of Trading Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities	$ 2,877,449	$ 2,625,436
Residential mortgage-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities	207,507	209,521
Commercial mortgage-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities	208,036	201,284
Other asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities	150,290	143,361
U.S. government-related securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities	36,991	47,067
Other government-related securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities	29,571	28,775
States, municipals, and political subdivisions
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities	288,085	293,791
Corporate securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities	1,850,471	1,590,936
Redeemable preferred stocks
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities	12,434	12,832
Fixed maturities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities	2,783,385	2,527,567
Equity securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities	16,742	6,656
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities	$ 77,322	$ 91,213