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	Jun. 30, 2018	Dec. 31, 2017
Held-to-maturity, Fair Value
Fair Value	$ 2,582,938	$ 2,776,327
Fixed maturities
Available-for-sale, Amortized Cost
Due in one year or less	930,606
Due after one year through five years	9,281,028
Due after five years through ten years	8,772,110
Due after ten years	31,950,015
Total	50,933,759
Available-for-sale, Fair Value
Due in one year or less	927,953
Due after one year through five years	9,123,063
Due after five years through ten years	8,545,593
Due after ten years	30,460,215
Total	49,056,824
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	2,677,726
Amortized Cost	2,677,726	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	2,582,938
Fair Value	$ 2,582,938	$ 2,776,327