Stock Options Granted and Exercised (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares subject to stock options granted	0	0	156,500	159,500
Weighted average grant date fair value per share	$ 0.00	$ 0.00	$ 31.36	$ 28.90
Shares subject to stock options exercised	93,911	42,780	197,965	103,310
Intrinsic value of stock options exercised	$ 2,299	$ 1,659	$ 4,788	$ 2,570
Proceeds received from stock options exercised	3,423	1,955	8,091	4,983
Tax benefits realized from stock options exercised	$ 733	$ 638	$ 1,597	$ 989