Calculation of net income per common share (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Net income	$ 10,558	$ 8,476
Basic:
Basic weighted average EPS shares	24,911	24,098
Basic net income per share	$ 0.42	$ 0.35
Diluted:
Basic weighted average EPS shares	24,911	24,098
Dilutive common stock equivalents	92	83
Dilutive EPS shares	25,003	24,181
Diluted net income per share	$ 0.42	$ 0.35
Anti-dilutive shares	242	641