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, 2019	Dec. 31, 2018
Available-for-sale, Amortized Cost
Amortized Cost	$ 52,656,572	$ 52,826,470
Available-for-sale, Fair Value
Fair Value	52,066,902	50,264,864
Held-to-maturity, Fair Value
Fair Value	2,594,441	2,547,210
Fixed maturities
Available-for-sale, Amortized Cost
Due in one year or less	1,259,571
Due after one year through five years	8,954,924
Due after five years through ten years	8,979,403
Due after ten years	32,679,663
Amortized Cost	51,873,561
Available-for-sale, Fair Value
Due in one year or less	1,257,644
Due after one year through five years	8,938,604
Due after five years through ten years	9,034,109
Due after ten years	32,053,534
Fair Value	51,283,891
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	2,607,356
Amortized Cost	2,607,356	2,633,474
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	2,594,441
Fair Value	$ 2,594,441	$ 2,547,210