Selected Quarterly Financial Data (Unaudited) - Summary of the Unaudited Quarterly Results of Operations (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
Quarterly Financial Information Disclosure [Abstract]
Net interest income	$ 47,319us-gaap_InterestIncomeExpenseNet	$ 46,514us-gaap_InterestIncomeExpenseNet	$ 45,489us-gaap_InterestIncomeExpenseNet	$ 42,029us-gaap_InterestIncomeExpenseNet	$ 41,921us-gaap_InterestIncomeExpenseNet	$ 40,712us-gaap_InterestIncomeExpenseNet	$ 40,630us-gaap_InterestIncomeExpenseNet	$ 40,256us-gaap_InterestIncomeExpenseNet	$ 181,351us-gaap_InterestIncomeExpenseNet	$ 163,519us-gaap_InterestIncomeExpenseNet	$ 164,815us-gaap_InterestIncomeExpenseNet
Provision for loan losses	1,840us-gaap_ProvisionForLoanAndLeaseLosses	(3,115)us-gaap_ProvisionForLoanAndLeaseLosses	3,129us-gaap_ProvisionForLoanAndLeaseLosses	1,218us-gaap_ProvisionForLoanAndLeaseLosses	454us-gaap_ProvisionForLoanAndLeaseLosses	(12)us-gaap_ProvisionForLoanAndLeaseLosses	516us-gaap_ProvisionForLoanAndLeaseLosses	300us-gaap_ProvisionForLoanAndLeaseLosses	3,072us-gaap_ProvisionForLoanAndLeaseLosses	1,258us-gaap_ProvisionForLoanAndLeaseLosses	3,100us-gaap_ProvisionForLoanAndLeaseLosses
Securities transactions	822us-gaap_GainLossOnSaleOfSecuritiesNet	284us-gaap_GainLossOnSaleOfSecuritiesNet	85us-gaap_GainLossOnSaleOfSecuritiesNet	450us-gaap_GainLossOnSaleOfSecuritiesNet	79us-gaap_GainLossOnSaleOfSecuritiesNet	90us-gaap_GainLossOnSaleOfSecuritiesNet	129us-gaap_GainLossOnSaleOfSecuritiesNet	122us-gaap_GainLossOnSaleOfSecuritiesNet	1,641us-gaap_GainLossOnSaleOfSecuritiesNet	420us-gaap_GainLossOnSaleOfSecuritiesNet	4,915us-gaap_GainLossOnSaleOfSecuritiesNet
Noninterest income	24,302us-gaap_NoninterestIncome	24,938us-gaap_NoninterestIncome	23,611us-gaap_NoninterestIncome	23,562us-gaap_NoninterestIncome	22,235us-gaap_NoninterestIncome	23,652us-gaap_NoninterestIncome	21,733us-gaap_NoninterestIncome	22,535us-gaap_NoninterestIncome	96,413us-gaap_NoninterestIncome	90,155us-gaap_NoninterestIncome	87,717us-gaap_NoninterestIncome
Noninterest expense	46,886us-gaap_NoninterestExpense	46,942us-gaap_NoninterestExpense	45,857us-gaap_NoninterestExpense	43,836us-gaap_NoninterestExpense	43,854us-gaap_NoninterestExpense	43,321us-gaap_NoninterestExpense	42,455us-gaap_NoninterestExpense	41,944us-gaap_NoninterestExpense	183,521us-gaap_NoninterestExpense	171,574us-gaap_NoninterestExpense	170,428us-gaap_NoninterestExpense
Net income	$ 15,749us-gaap_NetIncomeLoss	$ 18,793us-gaap_NetIncomeLoss	$ 14,688us-gaap_NetIncomeLoss	$ 14,657us-gaap_NetIncomeLoss	$ 13,861us-gaap_NetIncomeLoss	$ 14,491us-gaap_NetIncomeLoss	$ 12,593us-gaap_NetIncomeLoss	$ 13,372us-gaap_NetIncomeLoss	$ 63,887us-gaap_NetIncomeLoss	$ 54,317us-gaap_NetIncomeLoss	$ 51,900us-gaap_NetIncomeLoss
Net income per common share:
Basic	$ 1.02us-gaap_EarningsPerShareBasic	$ 1.22us-gaap_EarningsPerShareBasic	$ 0.94us-gaap_EarningsPerShareBasic	$ 0.96us-gaap_EarningsPerShareBasic	$ 0.91us-gaap_EarningsPerShareBasic	$ 0.94us-gaap_EarningsPerShareBasic	$ 0.83us-gaap_EarningsPerShareBasic	$ 0.88us-gaap_EarningsPerShareBasic	$ 4.14us-gaap_EarningsPerShareBasic	$ 3.56us-gaap_EarningsPerShareBasic	$ 3.42us-gaap_EarningsPerShareBasic
Diluted	$ 0.99us-gaap_EarningsPerShareDiluted	$ 1.19us-gaap_EarningsPerShareDiluted	$ 0.92us-gaap_EarningsPerShareDiluted	$ 0.94us-gaap_EarningsPerShareDiluted	$ 0.88us-gaap_EarningsPerShareDiluted	$ 0.93us-gaap_EarningsPerShareDiluted	$ 0.82us-gaap_EarningsPerShareDiluted	$ 0.86us-gaap_EarningsPerShareDiluted	$ 4.04us-gaap_EarningsPerShareDiluted	$ 3.49us-gaap_EarningsPerShareDiluted	$ 3.36us-gaap_EarningsPerShareDiluted

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The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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The net gain (loss) realized from the sale, exchange, redemption, or retirement of securities, not separately or otherwise categorized as trading, available-for-sale, or held-to-maturity.

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The net amount of operating interest income (expense).

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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The total amount of noninterest income which may be derived from: (1) fees and commissions; (2) premiums earned; (3) insurance policy charges; (4) the sale or disposal of assets; and (5) other sources not otherwise specified.

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Amount of expense related to estimated loss from loan and lease transactions.

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