Segment Information - Schedule of Financial Information Concerning Segments (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Segment Reporting Information [Line Items]
Net interest income	$ 90,993us-gaap_InterestIncomeExpenseNet	$ 83,478us-gaap_InterestIncomeExpenseNet
Noninterest income	55,295us-gaap_NoninterestIncome	40,563us-gaap_NoninterestIncome
Noncash items:
Depreciation and software amortization	4,848	3,392
Provision for loan losses	1us-gaap_ProvisionForLoanAndLeaseLosses	37us-gaap_ProvisionForLoanAndLeaseLosses
Amortization of intangibles	3,081us-gaap_AmortizationOfIntangibleAssets	1,837us-gaap_AmortizationOfIntangibleAssets
Income tax expense (benefit)	9,225us-gaap_IncomeTaxExpenseBenefit	9,242us-gaap_IncomeTaxExpenseBenefit
Segment profit	20,906us-gaap_NetIncomeLoss	26,510us-gaap_NetIncomeLoss
Segment assets	11,951,307us-gaap_Assets	9,544,780us-gaap_Assets	11,647,551us-gaap_Assets
Banking [Member]
Segment Reporting Information [Line Items]
Net interest income	93,078	83,554
Noninterest income	42,839	28,260
Noncash items:
Depreciation and software amortization	4,656	3,236
Provision for loan losses	1	37
Amortization of intangibles	2,605	1,430
Income tax expense (benefit)	9,297	11,224
Segment profit	24,783	23,501
Segment assets	11,804,609	9,412,745
Insurance [Member]
Segment Reporting Information [Line Items]
Net interest income	2	3
Noninterest income	11,987	11,976
Noncash items:
Depreciation and software amortization	34	35
Amortization of intangibles	476	407
Income tax expense (benefit)	615	803
Segment profit	964	1,873
Segment assets	60,700	63,180
Other [Member]
Segment Reporting Information [Line Items]
Net interest income	(2,087)	(79)
Noninterest income	469	327
Noncash items:
Depreciation and software amortization	158	121
Income tax expense (benefit)	(687)	(2,785)
Segment profit	(4,841)	1,136
Segment assets	$ 85,998	$ 68,855

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The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

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Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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The aggregate expense recognized in the current period that allocates the cost of tangible assets, intangible assets, or depleting assets to periods that benefit from use of the assets.

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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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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