Reinsurance - Additional Information (Detail) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2018 USD ($) Reinsurers	Sep. 20, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) Reinsurers	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) Reinsurers
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Ceded losses recognized as a reduction in losses and LAE		$ 27	$ 5	$ 58,493	$ 5
Reinsurance recoverable	$ 73,095			$ 73,095		$ 103,104
Number of reinsurers | Reinsurers	38			38		37
Percentage of reinsurance recoverable for paid losses and loss adjustment expenses major reinsurers				30.70%
Number of reinsurers in which major reinsurance recoverable concentrated | Reinsurers	2			2
Net increase in premiums ceded			12,465	$ 115	5,509
Net reduction in premiums ceded	$ 600
Other assets	24,701			24,701		$ 9,741
Oxbridge [Member]
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Net increase in premiums ceded	0		$ 2,415	448	$ 903
Other assets	0			0		479
Reinsurance [Member]
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Other assets	$ 2,858			$ 2,858		$ 2,393