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	931,606
Due after one year through five years	9,296,923
Due after five years through ten years	8,929,138
Due after ten years	31,980,596
Amortized Cost	51,138,263
Available-for-sale, Fair Value
Due in one year or less	928,953
Due after one year through five years	9,138,806
Due after five years through ten years	8,697,409
Due after ten years	30,490,156
Fair Value	49,255,324
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