Investments of Insurance Subsidiaries - Schedule of Maturities of Investments (Detail) $ in Millions	Sep. 30, 2017 USD ($)
Investments, Debt and Equity Securities [Abstract]
Due in one year or less, Amortized Cost	$ 100
Due after one year through five years, Amortized Cost	76
Due after five years through ten years, Amortized Cost	178
Due after ten years, Amortized Cost	50
Amortized Cost, Total	404
Due in one year or less, Fair Value	100
Due after one year through five years, Fair Value	80
Due after five years through ten years, Fair Value	187
Due after ten years, Fair Value	51
Fair Value, Total	$ 418