Fair Value Measurements and Investments Fair Value Measurements and Investments - Fair Value of Debt (Details) - USD ($) $ in Millions | Jun. 30, 2018 | Sep. 30, 2017 |
2017 Notes | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt Instrument, Interest Rate, Stated Percentage | | 1.20% |
Senior Notes [Member] | 2017 Notes | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt Instrument, Interest Rate, Stated Percentage | | 1.20% |
Senior Notes [Member] | 2020 Notes | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt Instrument, Interest Rate, Stated Percentage | 2.20% | |
Senior Notes [Member] | September 2022 Notes | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt Instrument, Interest Rate, Stated Percentage | 2.15% | |
Senior Notes [Member] | 2022 Notes | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt Instrument, Interest Rate, Stated Percentage | 2.80% | |
Senior Notes [Member] | 2025 Notes | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt Instrument, Interest Rate, Stated Percentage | 3.15% | |
Senior Notes [Member] | 2027 Notes | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt Instrument, Interest Rate, Stated Percentage | 2.75% | |
Senior Notes [Member] | 2035 Notes | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt Instrument, Interest Rate, Stated Percentage | 4.15% | |
Senior Notes [Member] | 2045 Notes | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt Instrument, Interest Rate, Stated Percentage | 4.30% | |
Senior Notes [Member] | 2047 Notes | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt Instrument, Interest Rate, Stated Percentage | 3.65% | |
Senior Notes [Member] | Carrying Amount | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Long-term Debt, Fair Value | $ 16,627 | $ 18,367 |
Senior Notes [Member] | Carrying Amount | 2017 Notes | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Long-term Debt, Fair Value | 0 | 1,749 |
Senior Notes [Member] | Carrying Amount | 2020 Notes | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Long-term Debt, Fair Value | 2,993 | 2,990 |
Senior Notes [Member] | Carrying Amount | September 2022 Notes | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Long-term Debt, Fair Value | 994 | 993 |
Senior Notes [Member] | Carrying Amount | 2022 Notes | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Long-term Debt, Fair Value | 2,241 | 2,240 |
Senior Notes [Member] | Carrying Amount | 2025 Notes | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Long-term Debt, Fair Value | 3,969 | 3,967 |
Senior Notes [Member] | Carrying Amount | 2027 Notes | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Long-term Debt, Fair Value | 741 | 740 |
Senior Notes [Member] | Carrying Amount | 2035 Notes | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Long-term Debt, Fair Value | 1,486 | 1,485 |
Senior Notes [Member] | Carrying Amount | 2045 Notes | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Long-term Debt, Fair Value | 3,463 | 3,463 |
Senior Notes [Member] | Carrying Amount | 2047 Notes | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Long-term Debt, Fair Value | 740 | 740 |
Senior Notes [Member] | Estimated Fair Value | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Long-term Debt, Fair Value | 16,577 | 19,171 |
Senior Notes [Member] | Estimated Fair Value | 2017 Notes | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Long-term Debt, Fair Value | 0 | 1,751 |
Senior Notes [Member] | Estimated Fair Value | 2020 Notes | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Long-term Debt, Fair Value | 2,951 | 3,031 |
Senior Notes [Member] | Estimated Fair Value | September 2022 Notes | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Long-term Debt, Fair Value | 959 | 997 |
Senior Notes [Member] | Estimated Fair Value | 2022 Notes | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Long-term Debt, Fair Value | 2,209 | 2,301 |
Senior Notes [Member] | Estimated Fair Value | 2025 Notes | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Long-term Debt, Fair Value | 3,869 | 4,098 |
Senior Notes [Member] | Estimated Fair Value | 2027 Notes | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Long-term Debt, Fair Value | 697 | 737 |
Senior Notes [Member] | Estimated Fair Value | 2035 Notes | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Long-term Debt, Fair Value | 1,554 | 1,637 |
Senior Notes [Member] | Estimated Fair Value | 2045 Notes | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Long-term Debt, Fair Value | 3,637 | 3,873 |
Senior Notes [Member] | Estimated Fair Value | 2047 Notes | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Long-term Debt, Fair Value | $ 701 | $ 746 |