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Loss and Loss Adjustment Expenses - Summary Exposures to Various Asbestos Related Claims (Detail) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Asbestos Claims [Line Items] |  |  |
Balance, beginning of period | $ 614,557 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense | $ 570,450 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense | $ 538,214 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense
Incurred losses and LAE during the current year | 244,916 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaims1 | 228,973 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaims1 | 219,903 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaims1
Loss and LAE payments | (52,848)us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseClaimsPaidCurrentYear1 | (51,169)us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseClaimsPaidCurrentYear1 | (50,423)us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseClaimsPaidCurrentYear1
Balance, end of period | 687,602 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense | 614,557 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense | 570,450 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense
Asbestos Issue [Member] |  |  |
Asbestos Claims [Line Items] |  |  |
Balance, beginning of period | 219 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis = us-gaap_AsbestosIssueMember | 167 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis = us-gaap_AsbestosIssueMember |
Incurred losses and LAE during the current year | 882 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaims1 / us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis = us-gaap_AsbestosIssueMember | 53 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaims1 / us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis = us-gaap_AsbestosIssueMember | 168 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaims1 / us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis = us-gaap_AsbestosIssueMember
Loss and LAE payments | (282)us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseClaimsPaidCurrentYear1 / us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis = us-gaap_AsbestosIssueMember | (1)us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseClaimsPaidCurrentYear1 / us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis = us-gaap_AsbestosIssueMember | (1)us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseClaimsPaidCurrentYear1 / us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis = us-gaap_AsbestosIssueMember
Balance, end of period | $ 819 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis = us-gaap_AsbestosIssueMember | $ 219 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis = us-gaap_AsbestosIssueMember | $ 167 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis = us-gaap_AsbestosIssueMember

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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 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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