Earnings per Share - Basic and Diluted Earnings Per Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share, Basic and Diluted [Abstract]
Numerator for basic and diluted earnings per share—income available to common shareholders	$ 5,190	$ 6,235	$ 5,080	$ 5,726	$ 4,922	$ 6,039	$ 4,830	$ 5,398	$ 22,231	$ 21,189	$ 20,872
Denominator for basic earnings per share—weighted-average shares outstanding									16,235	16,605	16,761
Effect of dilutive securities—employee stock options and awards									90	70	55
Denominator for diluted earnings per share—adjusted weighted-average shares outstanding									16,325	16,675	16,816
Basic earnings per share	$ 0.32	$ 0.38	$ 0.31	$ 0.35	$ 0.30	$ 0.36	$ 0.29	$ 0.32	$ 1.37	$ 1.28	$ 1.25
Diluted earnings per share	$ 0.32	$ 0.38	$ 0.31	$ 0.35	$ 0.30	$ 0.36	$ 0.29	$ 0.32	$ 1.36	$ 1.27	$ 1.24
Average anti-dilutive options and awards excluded from computation of diluted earnings per share									499	546	581