Calculation of net income per common share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Net income	$ 7,207	$ 8,296	$ 15,683	$ 15,587
Basic:
Basic weighted average EPS shares	24,393	24,091	24,098	24,072
Basic net income per share	$ 0.30	$ 0.34	$ 0.65	$ 0.65
Diluted:
Basic weighted average EPS shares	24,393	24,091	24,098	24,072
Dilutive common stock equivalents	31	39	83	51
Dilutive EPS shares	24,423	24,130	24,181	24,123
Diluted net income per share	$ 0.30	$ 0.34	$ 0.65	$ 0.65
Anti-dilutive shares	527	741	507	717