INVESTMENT OPERATIONS - Schedule of Fair Value of Trading Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities - trading	$ 2,531,538	$ 2,625,436
Residential mortgage-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities - trading	197,722	209,521
Commercial mortgage-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities - trading	181,870	201,284
Other asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities - trading	134,311	143,361
U.S. government-related securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities - trading	45,072	47,067
Other government-related securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities - trading	27,596	28,775
States, municipals, and political subdivisions
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities - trading	284,213	293,791
Corporate securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities - trading	1,552,707	1,590,936
Redeemable preferred stocks
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities - trading	10,015	12,832
Fixed maturities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities - trading	2,433,506	2,527,567
Equity securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities - trading	6,661	6,656
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities - trading	$ 91,371	$ 91,213