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, 2017	Dec. 31, 2016
Held-to-maturity Securities, Fair Value
Fair Value	$ 2,776,327	$ 2,733,340
Fixed maturities
Available-for-sale, Amortized Cost
Due in one year or less	723,923
Due after one year through five years	6,657,059
Due after five years through ten years	7,475,095
Due after ten years	23,633,285
Total	38,489,362
Available-for-sale Fair Value
Due in one year or less	723,435
Due after one year through five years	6,639,380
Due after five years through ten years	7,481,482
Due after ten years	23,667,566
Total	38,511,863
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	2,718,904
Amortized Cost	2,718,904	2,770,177
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	2,776,327
Fair Value	$ 2,776,327	$ 2,733,340