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	Sep. 30, 2018	Dec. 31, 2017
Available-for-sale, Amortized Cost
Amortized Cost	$ 52,559,863	$ 39,783,880
Available-for-sale, Fair Value
Fair Value	50,385,896	39,819,928
Held-to-maturity, Fair Value
Fair Value	2,577,841	2,776,327
Fixed maturities
Available-for-sale, Amortized Cost
Due in one year or less	957,984
Due after one year through five years	9,269,777
Due after five years through ten years	9,259,344
Due after ten years	32,596,570
Amortized Cost	52,083,675
Available-for-sale, Fair Value
Due in one year or less	955,779
Due after one year through five years	9,096,671
Due after five years through ten years	9,001,644
Due after ten years	30,855,614
Fair Value	49,909,708
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	2,653,605
Amortized Cost	2,653,605	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	2,577,841
Fair Value	$ 2,577,841	$ 2,776,327