Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Corporate income tax rate		35.00%	35.00%	35.00%
Deferred tax expense due to tax law changes		$ 17,100,000
Transition tax		$ 2,600,000
Tax Cuts and Jobs Act of 2017, accounting complete		false
Unrecognized tax benefits		$ 0	$ 0	$ 0
Deferred Tax Assets: State and Local		616,000	$ 699,000
Valuation allowance for deferred tax assets		$ 1,182,000
Scenario Forecast
Income Taxes [Line Items]
Corporate income tax rate	21.00%
United Kingdom
Income Taxes [Line Items]
Foreign effective income tax rate		35.00%
United Kingdom | Syndicate 1221 | Maximum
Income Taxes [Line Items]
Percentage of premiums		50.00%
United Kingdom | Syndicate 1221 | Minimum
Income Taxes [Line Items]
Foreign effective income tax rate		35.00%
State and Local Jurisdiction | Maximum
Income Taxes [Line Items]
State and local tax carry-forwards expiration year		2035
State and Local Jurisdiction | Minimum
Income Taxes [Line Items]
State and local tax carry-forwards expiration year		2027