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	Dec. 31, 2018	Dec. 31, 2017
Available-for-sale, Amortized Cost
Amortized Cost	$ 52,826,470	$ 39,783,880
Available-for-sale Fair Value
Total	50,264,864	39,819,928
Held-to-maturity Securities, Fair Value
Fair Value	2,547,210	2,776,327
Fixed maturities
Available-for-sale, Amortized Cost
Due in one year or less	1,146,730
Due after one year through five years	9,272,731
Due after five years through ten years	9,200,724
Due after ten years	32,429,928
Amortized Cost	52,050,113
Available-for-sale Fair Value
Due in one year or less	1,142,140
Due after one year through five years	9,120,056
Due after five years through ten years	8,947,546
Due after ten years	30,278,765
Total	49,488,507
Held-to-maturity Securities, 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	2,633,474
Amortized Cost	2,633,474	2,718,904
Held-to-maturity Securities, 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	2,547,210
Fair Value	$ 2,547,210	$ 2,776,327