Reinsurance - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Prepaid Reinsurance Premiums [Abstract]
Policyholder Benefits and Claims Incurred, Ceded	$ 1,094.2	$ 764.4	$ 457.3

Amount of reduction of provision for policy benefits and costs incurred for policies ceded.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 20
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=6480758&loc=d3e6811-158387

Name:	us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration