Calculation of net income per common share (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Net income	$ 10,990	$ 11,257	$ 26,673	$ 26,844
Basic:
Basic weighted average EPS shares	24,893	24,099	24,463	24,081
Basic net income per share	$ 0.44	$ 0.47	$ 1.09	$ 1.11
Diluted:
Basic weighted average EPS shares	24,893	24,099	24,463	24,081
Dilutive common stock equivalents	56	43	72	47
Dilutive EPS shares	24,949	24,142	24,535	24,128
Diluted net income per share	$ 0.44	$ 0.47	$ 1.09	$ 1.11
Anti-dilutive shares	459	697	489	716