Earnings per Common Share - Summary of Earnings per Common Share (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Numerator for both basic and diluted earnings per common share:
Net income	$ 17,017	$ 11,998	$ 33,037	$ 23,988
Denominator:
Total average basic common shares outstanding	29,245,201	26,647,050	29,228,355	26,637,537
Effect of dilutive stock options	63,605	3,275	59,663	3,342
Total diluted average common shares outstanding	29,308,806	26,650,325	29,288,018	26,640,879
Earnings per common share - basic	$ 0.58	$ 0.45	$ 1.13	$ 0.90
Earnings per common share - diluted	$ 0.58	$ 0.45	$ 1.13	$ 0.90