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, 2014		Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Earnings Per Share Basic And Diluted [Line Items]
Basic earnings per share, Weighted Average Shares Outstanding																	83,785		83,582		83,394
Potential dilutive effect of stock options, Weighted Average Shares Outstanding																	186		182		192
Diluted earnings per share, Weighted Average Shares Outstanding	84,055		83,968		83,916		83,944		83,900		83,776		83,700		83,678		83,971		83,764		83,586
Basic earnings per share, Earnings per Share																	$ 2.98		$ 2.63		$ 2.23
Potential dilutive effect of stock options, Earnings per Share																	$ (0.01)		$ (0.01)		$ (0.01)
Diluted earnings per share, Earnings per Share	$ 0.84	[1]	$ 0.73	[1]	$ 0.71	[1]	$ 0.69	[1]	$ 0.79	[1]	$ 0.71	[1]	$ 0.57	[1]	$ 0.55	[1]	$ 2.97	[1]	$ 2.62	[1]	$ 2.22

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