INVESTMENT OPERATIONS - Schedule of Fair Value of Trading Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Trading Securities [Line Items]
Trading securities	$ 2,592,645	$ 2,457,010
Residential mortgage-backed securities
Trading Securities [Line Items]
Trading securities	203,455	241,836
Commercial mortgage-backed securities
Trading Securities [Line Items]
Trading securities	204,445	188,925
Other asset-backed securities
Trading Securities [Line Items]
Trading securities	147,997	159,907
U.S. government-related securities
Trading Securities [Line Items]
Trading securities	51,080	59,794
Other government-related securities
Trading Securities [Line Items]
Trading securities	24,342	44,207
States, municipals, and political subdivisions
Trading Securities [Line Items]
Trading securities	293,560	286,413
Corporate securities
Trading Securities [Line Items]
Trading securities	1,565,561	1,423,833
Redeemable preferred stocks
Trading Securities [Line Items]
Trading securities	11,808	11,277
Fixed maturities
Trading Securities [Line Items]
Trading securities	2,502,248	2,416,192
Equity securities
Trading Securities [Line Items]
Trading securities	8,488	9,892
Short-term investments
Trading Securities [Line Items]
Trading securities	$ 81,909	$ 30,926