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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
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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 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense | $ 570,450 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense | $ 538,214 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense
Less amounts recoverable from reinsurers on unpaid loss and loss adjustment expenses | 48,699 us-gaap_ReinsuranceRecoverablesOnUnpaidLosses | 55,190 us-gaap_ReinsuranceRecoverablesOnUnpaidLosses | 60,937 us-gaap_ReinsuranceRecoverablesOnUnpaidLosses
Net balance, beginning of period | 565,858 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseNet | 515,260 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseNet | 477,277 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseNet
Add incurred related to: |  |  |
Current accident year | 268,633 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersCurrentYearClaimsAndClaimsAdjustmentExpense | 241,584 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersCurrentYearClaimsAndClaimsAdjustmentExpense | 222,393 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersCurrentYearClaimsAndClaimsAdjustmentExpense
Prior accident years | (23,717)us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense | (12,611)us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense | (2,490)us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense
Total incurred | 244,916 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaims1 | 228,973 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaims1 | 219,903 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaims1
Less paid related to: |  |  |
Current accident year | 52,848 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseClaimsPaidCurrentYear1 | 51,169 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseClaimsPaidCurrentYear1 | 50,423 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseClaimsPaidCurrentYear1
Prior accident years | 129,658 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseClaimsPaidPriorYears1 | 127,206 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseClaimsPaidPriorYears1 | 131,497 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseClaimsPaidPriorYears1
Total paid | 182,506 us-gaap_PaymentsForLossesAndLossAdjustmentExpense | 178,375 us-gaap_PaymentsForLossesAndLossAdjustmentExpense | 181,920 us-gaap_PaymentsForLossesAndLossAdjustmentExpense
Net balance, end of period | 628,268 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseNet | 565,858 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseNet | 515,260 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseNet
Add amounts recoverable from reinsurers on unpaid loss and loss adjustment expenses | 59,334 us-gaap_ReinsuranceRecoverablesOnUnpaidLosses | 48,699 us-gaap_ReinsuranceRecoverablesOnUnpaidLosses | 55,190 us-gaap_ReinsuranceRecoverablesOnUnpaidLosses
Balance, end of period | $ 687,602 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense | $ 614,557 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense | $ 570,450 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense

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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a),(b),(c) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 + Details
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- Definition Amount of payments to settle claims incurred in the current period and related claims settlement costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 + Details
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- Definition Amount of payments to settle claims incurred in prior periods and related claims settlement costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 + Details
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- Definition Amount of expense for claims incurred and costs incurred in the claim settlement process. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 + Details
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- Definition 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 Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Financial Reporting Release (FRR) -Number 403 -Paragraph 04 -Subparagraph b Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 13 -Subparagraph a -Article 7 + Details
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- Definition Amount of payments to settle insured claims and pay costs incurred in the claims settlement process. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 25 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3536-108585 + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -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 + Details
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- Definition Amount of expense for claims incurred in the current reporting period and related claims settlement costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e575016-122915 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 18 -Article 12 + Details
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- Definition Amount of expense for claims incurred in prior reporting periods and related claims settlement costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e575016-122915 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 18 -Article 12 + Details
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