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Loss and Loss Adjustment Expenses - Summary Exposures to Various Asbestos Related Claims (Detail) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Asbestos Claims [Line Items] |  |  |
Balance, beginning of period | $ 687,602 | $ 614,557 | $ 570,450
Incurred losses and LAE during the current year | 214,573 | 244,916 | 228,973
Loss and LAE payments | (53,955) | (52,848) | (51,169)
Balance, end of period | 718,033 | 687,602 | 614,557
Asbestos Issue [Member] |  |  |
Asbestos Claims [Line Items] |  |  |
Balance, beginning of period | 819 | 219 | 167
Incurred losses and LAE during the current year | 444 | 882 | 53
Loss and LAE payments | (305) | (282) | (1)
Balance, end of period | $ 958 | $ 819 | $ 219

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- Definition Asbestos Claims [Line Items] + References No definition available. + Details
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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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Period Type: | instant

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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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Namespace Prefix: | us-gaap_
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- Details
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Period Type: |