Schedule of Basic and Diluted Earnings Per Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Earnings Per Share Basic And Diluted [Line Items]
Basic earnings per share, Weighted Average Shares																	41,697		41,619		41,524
Potential dilutive effect of stock options outstanding, Weighted Average Shares																	96		98		87
Diluted earnings per share, Weighted Average Shares	41,833		41,802		41,774		41,765		41,736		41,724		41,696		41,711		41,793		41,717		41,611
Basic earnings per share, Earnings per Share																	$ 4.45		$ 2.95		$ 2.74
Potential dilutive effect of stock options outstanding, Earnings per Share																	$ (0.01)		$ (0.01)		$ (0.01)
Diluted earnings per share, Earnings Per Share	$ 1.19	[1]	$ 1.23	[1]	$ 1.04	[1]	$ 0.98	[1]	$ 1.05	[1]	$ 0.63	[1]	$ 0.66	[1]	$ 0.60	[1]	$ 4.44	[1]	$ 2.94	[1]	$ 2.73

[1] Earnings per share for the quarter are computed independently and may not equal the earnings per share computed for the total year.