Summary of Significant Accounting Policies - Computation of Basic EPS to Dilutive EPS (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Net earnings	$ 74,225	$ 68,369	$ 59,659
Effect of stock options, Net Earnings
Net earnings per share, assuming dilution, Net Earnings	$ 74,225	$ 68,369	$ 59,659
Net earnings per share, basic, Weighted Average Shares	31,480,155	31,443,712	31,291,486
Effect of stock options, Weighted Average Shares	21,112	18,252	28,077
Net earnings per share, assuming dilution, Weighted Average Shares	31,501,267	31,461,964	31,319,563
Net earnings per share, basic, Per Share Amount	$ 2.36	$ 2.17	$ 1.91
Effect of stock options, Per Share Amount
Net earnings per share, assuming dilution, Per Share Amount	$ 2.36	$ 2.17	$ 1.91