Calculation for Basic and Diluted Earnings per Common Share (Detail) (USD $) In Thousands, except 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
Calculation For Basic And Diluted Earnings Per Common Share [Line Items]
Net income	$ 5,715us-gaap_NetIncomeLoss	$ 7,043us-gaap_NetIncomeLoss	$ 7,007us-gaap_NetIncomeLoss	$ 5,725us-gaap_NetIncomeLoss	$ 5,324us-gaap_NetIncomeLoss	$ 5,412us-gaap_NetIncomeLoss	$ 5,435us-gaap_NetIncomeLoss	$ 7,141us-gaap_NetIncomeLoss	$ 25,490us-gaap_NetIncomeLoss	$ 23,312us-gaap_NetIncomeLoss	$ 28,577us-gaap_NetIncomeLoss
Dividends on preferred stock	227	228	227	228	252	261	253	258	910	1,024	1,058
Net income available to common shareholders	$ 5,488	$ 6,815	$ 6,780	$ 5,497	$ 5,072	$ 5,151	$ 5,182	$ 6,883	$ 24,580	$ 22,288	$ 27,519
Weighted average number of common shares outstanding, basic	18,403,959	18,402,764	18,395,996	18,423,123	19,136,317	20,008,861	19,997,991	20,032,694	18,406,363	19,792,099	19,127,065
Convertible preferred stock									1,046,175	1,132,998	1,285,848
Contingently issuable shares									11,448	12,352
Weighted average number of common shares outstanding, diluted	19,482,000	19,466,126	19,457,237	19,506,647	20,233,737	21,123,788	21,205,078	21,258,490	19,483,054	20,961,800	20,419,569
Basic earnings per common share	$ 0.30us-gaap_EarningsPerShareBasic	$ 0.37us-gaap_EarningsPerShareBasic	$ 0.37us-gaap_EarningsPerShareBasic	$ 0.30us-gaap_EarningsPerShareBasic	$ 0.27us-gaap_EarningsPerShareBasic	$ 0.26us-gaap_EarningsPerShareBasic	$ 0.26us-gaap_EarningsPerShareBasic	$ 0.34us-gaap_EarningsPerShareBasic	$ 1.34us-gaap_EarningsPerShareBasic	$ 1.13us-gaap_EarningsPerShareBasic	$ 1.44us-gaap_EarningsPerShareBasic
Diluted earnings per common share	$ 0.29us-gaap_EarningsPerShareDiluted	$ 0.36us-gaap_EarningsPerShareDiluted	$ 0.36us-gaap_EarningsPerShareDiluted	$ 0.29us-gaap_EarningsPerShareDiluted	$ 0.26us-gaap_EarningsPerShareDiluted	$ 0.26us-gaap_EarningsPerShareDiluted	$ 0.26us-gaap_EarningsPerShareDiluted	$ 0.34us-gaap_EarningsPerShareDiluted	$ 1.31us-gaap_EarningsPerShareDiluted	$ 1.11us-gaap_EarningsPerShareDiluted	$ 1.40us-gaap_EarningsPerShareDiluted
Total potential antidilutive shares									222,651	317,691	420,802
Stock Option
Calculation For Basic And Diluted Earnings Per Common Share [Line Items]
Stock attributable to share based compensation									18,607	19,337	4,549
Total potential antidilutive shares									222,651	317,420	420,802
Restricted Stock
Calculation For Basic And Diluted Earnings Per Common Share [Line Items]
Stock attributable to share based compensation									461	5,014	2,107
Total potential antidilutive shares										271

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Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

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