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	777,460
Due after one year through five years	6,292,741
Due after five years through ten years	7,685,870
Due after ten years	23,653,488
Amortized Cost	38,409,559
Available-for-sale, Fair Value
Due in one year or less	778,156
Due after one year through five years	6,325,294
Due after five years through ten years	7,718,256
Due after ten years	23,275,031
Fair Value	38,096,737
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