Earnings Per Share - Computation of Basic and Diluted Earnings Per Common Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 19,343	$ 12,629	$ 34,669	$ 26,122
Basic weighted average shares outstanding	36,868	36,128	36,821	36,531
Basic earnings per share	$ 0.52	$ 0.35	$ 0.94	$ 0.72
Dilutive effect of stock options and restricted stock	951	1,312	925	1,536
Diluted weighted average shares outstanding	37,819	37,440	37,746	38,067
Diluted earnings per share	$ 0.51	$ 0.34	$ 0.92	$ 0.69
Common Stock Voting [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Basic weighted average shares outstanding	36,868	36,128	36,821	35,702
Common Stock Non-Voting [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Basic weighted average shares outstanding				829