Fair Value - Additional Information (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair Value Disclosures [Line Items]
Fair value reinsurement rate	100.00%
Quoted prices in active markets for identical assets (Level 1)
Fair Value Disclosures [Line Items]
Fair value measurement of investment	$ 49.3	$ 52.1

Fair value portion of liability related to investment contracts for example, but not limited to, guaranteed investment contract or annuities.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 825
-Section 25
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6487514&loc=d3e32427-158581

Name:	us-gaap_LiabilitiesRelatedToInvestmentContractsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant