Earnings Per Share - Computation of Basic and Diluted Earnings Per Common Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share Basic [Line Items]
Net income from continuing operations	$ 74,806	$ 68,563	$ 61,784
Net income (loss) from discontinued operations		7,453	(1,715)
Net income	$ 74,806	$ 76,016	$ 60,069
Basic weighted average shares outstanding	36,930	36,886	36,516
Income from continuing operations	$ 2.03	$ 1.86	$ 1.69
Income (loss) from discontinued operations		$ 0.20	$ (0.04)
Net income per common share	$ 2.03	$ 2.06	$ 1.65
Dilutive effect of stock options and restricted stock	959	1,002	1,300
Diluted weighted average shares outstanding	37,889	37,888	37,816
Income from continuing operations	$ 1.97	$ 1.81	$ 1.64
Income (loss) from discontinued operations		$ 0.20	$ (0.05)
Net income per common share	$ 1.97	$ 2.01	$ 1.59
Common Stock Voting [Member]
Earnings Per Share Basic [Line Items]
Basic weighted average shares outstanding	36,930	36,886	36,101
Common Stock Non-Voting [Member]
Earnings Per Share Basic [Line Items]
Basic weighted average shares outstanding			415