Summary of Stock Option Activity (Detail) (USD $)	12 Months Ended
	Dec. 31, 2011 Year	Dec. 31, 2010 Year	Dec. 31, 2009 Year	Dec. 31, 2008 Year
Shares Subject To Options
Beginning Balance	805,184	841,851	953,772
Options granted			102,800
Options exercised	(172,384)	(33,767)	(213,721)
Options canceled		(2,900)	(1,000)
Ending Balance	632,800	805,184	841,851
Exercisable at end of year	632,800
Weighted Average Exercise Price Per Share
Beginning Balance	$ 40.32	$ 39.94	$ 34.92
Options granted			$ 37.70
Options exercised	$ 26.28	$ 32.77	$ 43.34
Options canceled			$ 15.69
Ending Balance	$ 44.14	$ 40.32	$ 39.94
Exercisable at end of year	$ 44.14
Weighted Average Outstanding Contractual Life (in years)
Weighted Average Outstanding Contractual Life (in years)	5.0	5.1	6.0	5.4
Weighted Average Outstanding Contractual Life (in years)	5.0