Investments of Insurance Subsidiaries - Schedule of Maturities of Investments (Detail) $ in Millions	Dec. 31, 2017 USD ($)
Investments, Debt and Equity Securities [Abstract]
Due in one year or less, Amortized Cost	$ 31
Due after one year through five years, Amortized Cost	84
Due after five years through ten years, Amortized Cost	187
Due after ten years, Amortized Cost	59
Amortized Cost, Total	361
Due in one year or less, Fair Value	31
Due after one year through five years, Fair Value	86
Due after five years through ten years, Fair Value	194
Due after ten years, Fair Value	60
Fair Value, Total	$ 371