INVESTMENT OPERATIONS - Schedule of Fair Value of Trading Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Trading Securities [Line Items]
Trading securities	$ 2,642,144	$ 2,457,010
Residential mortgage-backed securities
Trading Securities [Line Items]
Trading securities	202,963	241,836
Commercial mortgage-backed securities
Trading Securities [Line Items]
Trading securities	208,935	188,925
Other asset-backed securities
Trading Securities [Line Items]
Trading securities	137,732	159,907
U.S. government-related securities
Trading Securities [Line Items]
Trading securities	48,112	59,794
Other government-related securities
Trading Securities [Line Items]
Trading securities	26,792	44,207
States, municipals, and political subdivisions
Trading Securities [Line Items]
Trading securities	300,332	286,413
Corporate securities
Trading Securities [Line Items]
Trading securities	1,599,453	1,423,833
Redeemable preferred stocks
Trading Securities [Line Items]
Trading securities	12,195	11,277
Fixed maturities
Trading Securities [Line Items]
Trading securities	2,536,514	2,416,192
Equity securities
Trading Securities [Line Items]
Trading securities	6,647	9,892
Short-term investments
Trading Securities [Line Items]
Trading securities	$ 98,983	$ 30,926