Long-Term Debt (Parenthetical) (Detail)	6 Months Ended
	Jun. 30, 2013
Medium-Term Notes
Debt Instrument [Line Items]
Debt instrument maturity date, month and year	2013-11
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 due November 2016
Debt Instrument [Line Items]
Debt instrument maturity date, month and year	2016-11
2011 Senior Notes due November 2041
Debt Instrument [Line Items]
Debt instrument maturity date, month and year	2041-11
2013 Senior Notes due March 2018
Debt Instrument [Line Items]
Debt instrument maturity date, month and year	2018-03
2013 Senior Notes due March 2023
Debt Instrument [Line Items]
Debt instrument maturity date, month and year	2023-03