Loss and Loss Adjustment Expenses - Reconciliation of Beginning and Ending Reserve Balances, Net of Related Amounts Recoverable from Reinsurers (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Liability for Unpaid Claims and Claims Adjustment Expense, Activity in Liability [Abstract]
Balance, beginning of period	$ 614,557	$ 570,450	$ 538,214
Less amounts recoverable from reinsurers on unpaid loss and loss adjustment expenses	48,699	55,190	60,937
Net balance, beginning of period	565,858	515,260	477,277
Add incurred related to:
Current accident year	268,633	241,584	222,393
Prior accident years	(23,717)	(12,611)	(2,490)
Total incurred	244,916	228,973	219,903
Less paid related to:
Current accident year	52,848	51,169	50,423
Prior accident years	129,658	127,206	131,497
Total paid	182,506	178,375	181,920
Net balance, end of period	628,268	565,858	515,260
Add amounts recoverable from reinsurers on unpaid loss and loss adjustment expenses	59,334	48,699	55,190
Balance, end of period	$ 687,602	$ 614,557	$ 570,450

Loss and Loss Adjustment Expenses - Reconciliation of Beginning and Ending Reserve Balances, Net of Related Amounts Recoverable from Reinsurers (Detail) (USD $)
In Thousands, unless otherwise specified

12 Months Ended

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Liability for Unpaid Claims and Claims Adjustment Expense, Activity in Liability [Abstract]

Balance, beginning of period

$ 614,557us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense

$ 570,450us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense

$ 538,214us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense

Less amounts recoverable from reinsurers on unpaid loss and loss adjustment expenses

48,699us-gaap_ReinsuranceRecoverablesOnUnpaidLosses

55,190us-gaap_ReinsuranceRecoverablesOnUnpaidLosses

60,937us-gaap_ReinsuranceRecoverablesOnUnpaidLosses

Net balance, beginning of period

565,858us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseNet

515,260us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseNet

477,277us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseNet

Add incurred related to:

Current accident year

268,633us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersCurrentYearClaimsAndClaimsAdjustmentExpense

241,584us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersCurrentYearClaimsAndClaimsAdjustmentExpense

222,393us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersCurrentYearClaimsAndClaimsAdjustmentExpense

Prior accident years

(23,717)us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense

(12,611)us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense

(2,490)us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense

Total incurred

244,916us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaims1

228,973us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaims1

219,903us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaims1

Less paid related to:

Current accident year

52,848us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseClaimsPaidCurrentYear1

51,169us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseClaimsPaidCurrentYear1

50,423us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseClaimsPaidCurrentYear1

Prior accident years

129,658us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseClaimsPaidPriorYears1

127,206us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseClaimsPaidPriorYears1

131,497us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseClaimsPaidPriorYears1

Total paid

182,506us-gaap_PaymentsForLossesAndLossAdjustmentExpense

178,375us-gaap_PaymentsForLossesAndLossAdjustmentExpense

181,920us-gaap_PaymentsForLossesAndLossAdjustmentExpense

Net balance, end of period

628,268us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseNet

565,858us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseNet

515,260us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseNet

Add amounts recoverable from reinsurers on unpaid loss and loss adjustment expenses

59,334us-gaap_ReinsuranceRecoverablesOnUnpaidLosses

48,699us-gaap_ReinsuranceRecoverablesOnUnpaidLosses

55,190us-gaap_ReinsuranceRecoverablesOnUnpaidLosses

Balance, end of period

$ 687,602us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense

$ 614,557us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense

$ 570,450us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense

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The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date.

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Amount of payments to settle claims incurred in the current period and related claims settlement costs.

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Amount of payments to settle claims incurred in prior periods and related claims settlement costs.

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Amount of expense for claims incurred and costs incurred in the claim settlement process.

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Liability as of the balance sheet date for amounts representing estimated cost of settling unpaid claims under the terms of the underlying insurance policies, less estimated reinsurance recoveries on such claims. This includes an estimate for claims which have been incurred but not reported. Claim adjustment expenses represent the costs estimated to be incurred in the settlement of unpaid claims.

+ References

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-Number 210

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-Paragraph 13

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 25

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Amount after valuation allowance of reinsurance recoverables for losses reported to the ceding insurer but not yet paid, amounts expected of incurred losses and settlement expenses, which have not yet been reported to the ceding insurer.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 310

-Section 45

-Paragraph 6

-URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21412-158489

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Amount of expense for claims incurred in the current reporting period and related claims settlement costs.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 235

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-Name Regulation S-X (SX)

-Number 210

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Amount of expense for claims incurred in prior reporting periods and related claims settlement costs.

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