Debt - Summary of Long-Term Debt (Parenthetical) (Detail)	12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011
Maturities Of Long Term Debt [Line Items]
Average year-end interest rate on domestic notes due through 2014	4.89%	1.05%
4.55% Notes due April 15, 2013 [Member]
Maturities Of Long Term Debt [Line Items]
Interest rates of debt instruments	4.55%
1.75% Notes due November 8, 2016 [Member]
Maturities Of Long Term Debt [Line Items]
Interest rates of debt instruments	1.75%
4.90% Notes due April 15, 2018 [Member]
Maturities Of Long Term Debt [Line Items]
Interest rates of debt instruments	4.90%
5.00% Notes due May 15, 2019 [Member]
Maturities Of Long Term Debt [Line Items]
Interest rates of debt instruments	5.00%
3.25% Notes due November 12, 2020 [Member]
Maturities Of Long Term Debt [Line Items]
Interest rates of debt instruments	3.25%
3.125% Notes due November 8, 2021 [Member]
Maturities Of Long Term Debt [Line Items]
Interest rates of debt instruments	3.125%
7.00% Debentures due August 1, 2027 [Member]
Maturities Of Long Term Debt [Line Items]
Interest rates of debt instruments	7.00%
6.70% Debentures due August 1, 2028 [Member]
Maturities Of Long Term Debt [Line Items]
Interest rates of debt instruments	6.70%
6.00% Notes due May 15, 2039 [Member]
Maturities Of Long Term Debt [Line Items]
Interest rates of debt instruments	6.00%
5.00% Notes due November 12, 2040 [Member]
Maturities Of Long Term Debt [Line Items]
Interest rates of debt instruments	5.00%