Investments - Available-for-Sale Fixed Maturity Securities by Contractual Maturity (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Cost or Amortized Cost, Due in one year or less	$ 1,374	$ 1,779
Cost or Amortized Cost, Due after one year through five years	7,931	7,566
Cost or Amortized Cost, Due after five years through ten years	15,853	15,892
Cost or Amortized Cost, Due after ten years	13,652	13,112
Cost or Amortized Cost, Total	38,810	38,349
Estimated Fair Value, Due in one year or less	1,404	1,828
Estimated Fair Value, Due after one year through five years	8,293	7,955
Estimated Fair Value, Due after five years through ten years	16,574	16,332
Estimated Fair Value, Due after ten years	15,815	14,778
Estimated Fair Value, Total	$ 42,086	$ 40,893