INVESTMENT OPERATIONS - Schedule of Fair Value of Trading Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Trading Securities [Line Items]
Trading securities	$ 2,537,299	$ 2,457,010
Residential mortgage-backed securities
Trading Securities [Line Items]
Trading securities	213,259	241,836
Commercial mortgage-backed securities
Trading Securities [Line Items]
Trading securities	209,482	188,925
Other asset-backed securities
Trading Securities [Line Items]
Trading securities	149,541	159,907
U.S. government-related securities
Trading Securities [Line Items]
Trading securities	59,627	59,794
Other government-related securities
Trading Securities [Line Items]
Trading securities	23,640	44,207
States, municipals, and political subdivisions
Trading Securities [Line Items]
Trading securities	292,796	286,413
Corporate securities
Trading Securities [Line Items]
Trading securities	1,533,256	1,423,833
Redeemable preferred stocks
Trading Securities [Line Items]
Trading securities	11,860	11,277
Fixed maturities
Trading Securities [Line Items]
Trading securities	2,493,461	2,416,192
Equity securities
Trading Securities [Line Items]
Trading securities	9,207	9,892
Short-term investments
Trading Securities [Line Items]
Trading securities	$ 34,631	$ 30,926