Loss and LAE Reserve Summary and Reconciliation to Consolidated Balance Sheet (Parenthetical) (Detail)	Dec. 31, 2016
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unallocated LAE as a percentage of gross loss and LAE reserves	1.30%

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Name:	us-gaap_ShortdurationInsuranceContractsReconciliationOfClaimsDevelopmentToLiabilityLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration