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, 2017	Dec. 31, 2016
Held-to-maturity, Fair Value
Fair Value	$ 2,796,603	$ 2,733,340
Fixed maturities
Available-for-sale, Amortized Cost
Due in one year or less	772,655
Due after one year through five years	6,278,517
Due after five years through ten years	7,537,131
Due after ten years	23,618,477
Total	38,206,780
Available-for-sale, Fair Value
Due in one year or less	773,331
Due after one year through five years	6,310,854
Due after five years through ten years	7,567,040
Due after ten years	23,238,933
Total	37,890,158
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	2,730,995
Amortized Cost	2,730,995	2,770,177
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	2,796,603
Fair Value	$ 2,796,603	$ 2,733,340