Long-Term Debt (Parenthetical) (Detail)	1 Months Ended	9 Months Ended
	Nov. 30, 2011	Sep. 30, 2012
Medium-Term Notes
Debt Instrument [Line Items]
Debt instrument maturity date, month and year		2013-11
2008 Senior Notes
Debt Instrument [Line Items]
Debt instrument maturity date, month and year		2013-03
2010 Senior Notes due October 2020
Debt Instrument [Line Items]
Debt instrument maturity date, month and year		2020-10
2010 Senior Notes due October 2040
Debt Instrument [Line Items]
Debt instrument maturity date, month and year		2040-10
2011 Senior Notes | Unsecured Senior Notes 2.50% Due November 1, 2016
Debt Instrument [Line Items]
Debt instrument maturity date	Nov. 01, 2016	Nov. 01, 2016
2011 Senior Notes | Unsecured Senior Notes 5.45% Due November 1, 2041
Debt Instrument [Line Items]
Debt instrument maturity date	Nov. 01, 2041	Nov. 01, 2041