Long-term Debt (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012
2.50% notes, due 2017, net of unamortized discount of $ 76 at December 31, 2012
Debt Instrument [Line Items]
Notes payable, unamortized discount	$ 76
3.70% notes, due 2022, net of unamortized discount of $ 2,395 at December 31, 2012
Debt Instrument [Line Items]
Notes payable, unamortized discount	2,415
4.00% notes, due 2022, net of unamortized discount of $ 1,020 at December 31, 2012
Debt Instrument [Line Items]
Notes payable, unamortized discount	1,004
7.05% notes, due 2029, net of unamortized discount of $ 1,538 at December 31, 2012
Debt Instrument [Line Items]
Notes payable, unamortized discount	1,523
5.125% notes, due 2042, net of unamortized discount of $ 904 at December 31, 2012
Debt Instrument [Line Items]
Notes payable, unamortized discount	904
6.375% notes, due 2012, net of unamortized discount of $ 38 at December 31, 2012
Debt Instrument [Line Items]
Notes payable, unamortized discount	$ 38