Reinsurance - Additional Information (Detail)	1 Months Ended		4 Months Ended	12 Months Ended
	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2017 USD ($) Reinsurers	Dec. 31, 2017 USD ($) Reinsurers	Dec. 31, 2016 USD ($) Reinsurers	Dec. 31, 2015 USD ($)
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Percentage of ceding commission on ceded premiums written			30.00%	30.00%
Excess of loss reinsurance contract period				3 years
Reinsurance recoverable			$ 100,760,000	$ 100,760,000	$ 0	$ 0
Number of reinsurers | Reinsurers			37	37	35
Recoveries pertaining to reinsurance contracts				$ 103,104,000	$ 0
Number of reinsurers in which major reinsurance recoverable concentrated | Reinsurers			2	2
Percentage of reinsurance recoverable major reinsurers				29.70%
Percentage of assumed premiums to net premium earned				4.90%	2.50%	22.00%
Credit risk associated with reinsurance receivables			$ 0	$ 0
Ceded losses recognized as a reduction in losses and LAE				214,082,000
Net reduction in premiums ceded					$ 12,677,000	$ 18,322,000
Net increase in premiums ceded				5,740,000
Amount received from retrospective reinsurance contracts	$ 5,716,000	$ 37,800,000
Other assets			10,550,000	10,550,000	11,342,000
Prepaid reinsurance premiums			22,286,000	22,286,000	24,554,000
Oxbridge [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Ceded losses recognized as a reduction in losses and LAE				7,400,000
Net reduction in premiums ceded					1,929,000	$ 2,797,000
Net increase in premiums ceded				933,000
Amount received from retrospective reinsurance contracts		$ 7,560,000
Other assets			479,000	479,000	1,043,000
Prepaid reinsurance premiums			0	0	338,000
Reinsurance [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Other assets			2,393,000	2,393,000	5,810,000
Prepaid reinsurance premiums			$ 0	$ 0	$ 2,152,000