Investments - Available-for-Sale Fixed Maturity Securities by Contractual Maturity (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Cost or Amortized Cost, Due in one year or less	$ 1,350	$ 1,135
Cost or Amortized Cost, Due after one year through five years	7,979	8,165
Cost or Amortized Cost, Due after five years through ten years	16,859	16,060
Cost or Amortized Cost, Due after ten years	11,893	12,852
Cost or Amortized Cost, Total	38,081	38,212
Estimated Fair Value, Due in one year or less	1,359	1,157
Estimated Fair Value, Due after one year through five years	8,139	8,501
Estimated Fair Value, Due after five years through ten years	16,870	16,718
Estimated Fair Value, Due after ten years	13,174	15,108
Estimated Fair Value, Total	$ 39,542	$ 41,484