INVESTMENT OPERATIONS - Schedule of Amortized Cost and Fair Value of Available-for-Sale and Held-to-Maturity Fixed Maturities, by Expected Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Held-to-maturity, Fair Value
Fair Value	$ 2,674,129	$ 2,776,327
Fixed maturities
Available-for-sale, Amortized Cost
Due in one year or less	909,921
Due after one year through five years	7,011,907
Due after five years through ten years	6,730,814
Due after ten years	23,722,202
Total	38,374,844
Available-for-sale, Fair Value
Due in one year or less	906,744
Due after one year through five years	6,900,780
Due after five years through ten years	6,573,090
Due after ten years	22,854,858
Total	37,235,472
Held-to-maturity, Amortized Cost
Due in one year or less	0
Due after one year through five years	0
Due after five years through ten years	0
Due after ten years	2,699,826
Amortized Cost	2,699,826	2,718,904
Held-to-maturity, Fair Value
Due in one year or less	0
Due after one year through five years	0
Due after five years through ten years	0
Due after ten years	2,674,129
Fair Value	$ 2,674,129	$ 2,776,327