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, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Earnings Per Share Basic And Diluted [Line Items]
Basic earnings per share, Weighted Average Shares																	41,791		41,697		41,619
Potential dilutive effect of stock options outstanding, Weighted Average Shares																	91		96		98
Diluted earnings per share, Weighted Average Shares	41,950		41,888		41,850		41,839		41,833		41,802		41,774		41,765		41,882		41,793		41,717
Basic earnings per share, Earnings per Share																	$ 5.26		$ 4.45		$ 2.95
Potential dilutive effect of stock options outstanding, Earnings per Share																	$ (0.02)		$ (0.01)		$ (0.01)
Diluted earnings per share, Earnings Per Share	$ 1.58	[1]	$ 1.42	[1]	$ 1.14	[1]	$ 1.11	[1]	$ 1.19	[1]	$ 1.23	[1]	$ 1.04	[1]	$ 0.98	[1]	$ 5.24	[1]	$ 4.44	[1]	$ 2.94

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