Fair Value Measurements and Investments - Additional Information (Detail) - USD ($) |
Feb. 01, 2019 |
Sep. 30, 2019 |
Sep. 30, 2018 |
|---|---|---|---|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | |||
| Fair value, assets, level 1 to level 2 transfers, amount | $ 0 | ||
| Long-term debt securities | 2,157,000,000 | $ 4,082,000,000 | |
| Goodwill and intangible asset impairment | $ 0 | ||
| Carrying value of long-term debt | 16,729,000,000 | 16,630,000,000 | |
| Senior Notes | |||
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | |||
| Carrying value of long-term debt | 16,700,000,000 | 16,600,000,000 | |
| Estimate of Fair Value Measurement | Senior Notes | |||
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | |||
| Estimated fair value | $ 18,400,000,000 | $ 16,600,000,000 |