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, 2018	Dec. 31, 2017
Held-to-maturity, Fair Value
Fair Value	$ 2,674,129	$ 2,776,327
Fixed maturities
Available-for-sale, Amortized Cost
Due in one year or less	912,935
Due after one year through five years	7,026,832
Due after five years through ten years	6,881,555
Due after ten years	23,756,852
Amortized Cost	38,578,174
Available-for-sale, Fair Value
Due in one year or less	909,757
Due after one year through five years	6,915,584
Due after five years through ten years	6,721,386
Due after ten years	22,889,681
Fair Value	37,436,408
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	2,699,826
Amortized Cost	2,699,826	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	2,674,129
Fair Value	$ 2,674,129	$ 2,776,327