Income Taxes - Income Tax Expense from Operations from the Amount Computed by Applying the U.S. Federal Income Tax Statutory Rate of 35% to Income Before Income Taxes (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Income tax computed at federal statutory tax rate	$ 25,812	$ 20,721	$ 13,000
Tax-exempt interest, net	(5,620)	(5,458)	(5,333)
State income tax	(146)	504	302
Dividends received deduction	(60)	(113)	(79)
Other	97	(87)	(100)
Income tax expense, Total	$ 20,083us-gaap_IncomeTaxExpenseBenefit	$ 15,567us-gaap_IncomeTaxExpenseBenefit	$ 7,790us-gaap_IncomeTaxExpenseBenefit

Income Taxes - Income Tax Expense from Operations from the Amount Computed by Applying the U.S. Federal Income Tax Statutory Rate of 35% to Income Before Income Taxes (Detail) (USD $)
In Thousands, unless otherwise specified

12 Months Ended

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Income Tax Disclosure [Abstract]

Income tax computed at federal statutory tax rate

$ 25,812us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate

$ 20,721us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate

$ 13,000us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate

Tax-exempt interest, net

(5,620)us-gaap_IncomeTaxReconciliationTaxExemptIncome

(5,458)us-gaap_IncomeTaxReconciliationTaxExemptIncome

(5,333)us-gaap_IncomeTaxReconciliationTaxExemptIncome

State income tax

(146)us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes

504us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes

302us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes

Dividends received deduction

(60)us-gaap_IncomeTaxReconciliationDeductionsDividends

(113)us-gaap_IncomeTaxReconciliationDeductionsDividends

(79)us-gaap_IncomeTaxReconciliationDeductionsDividends

Other

97us-gaap_IncomeTaxReconciliationOtherReconcilingItems

(87)us-gaap_IncomeTaxReconciliationOtherReconcilingItems

(100)us-gaap_IncomeTaxReconciliationOtherReconcilingItems

Income tax expense, Total

$ 20,083us-gaap_IncomeTaxExpenseBenefit

$ 15,567us-gaap_IncomeTaxExpenseBenefit

$ 7,790us-gaap_IncomeTaxExpenseBenefit

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-08.(h))

-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 08

-Paragraph h

-Article 4

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Income Tax Expense (or Benefit)

-URI http://asc.fasb.org/extlink&oid=6515339

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 9

-Subparagraph (a),(b)

-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32639-109319

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 12

-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32687-109319

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 13

-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32698-109319

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The amount of income tax expense or benefit for the period computed by applying the domestic federal statutory tax rates to pretax income from continuing operations.

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- Definition

Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to tax exempt income, equity in earnings (loss) of an unconsolidated subsidiary, minority noncontrolling interest income (loss), tax holiday, disposition of a business, disposition of an asset, repatriation of foreign earnings, repatriation of foreign earnings jobs creation act of 2004, increase (decrease) in enacted tax rate, prior year income taxes, increase (decrease) in deferred tax asset valuation allowance, and other adjustments.

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