Income Taxes [Line Items]

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Non-U.S. insurance income constituting taxable income under Sub Part F of the IRC.

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Statutory rate on the foreign source insurance income and foreign tax credit.

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Tax cuts and jobs act of 2017, incomplete accounting, provisional transition tax.

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- Definition

Tax cuts and jobs act of 2017 tax benefit expense related to remeasurement of deferred tax assets.

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Tax cuts and jobs act of 2017 tax benefit (expense) related to transition tax.

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- Definition

Amount of interest expense for an underpayment of income taxes , net of tax.

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- Definition

Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from state taxes.

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Amount of unrecognized tax benefits.

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Amount of expense for penalties related to a tax position claimed or expected to be claimed in the tax return.

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Amount of interest expense for an underpayment of income taxes.

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Amount of increase (decrease) in the valuation allowance for a specified deferred tax asset.

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Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Corporate income tax rate			21.00%	35.00%	35.00%
Transition tax		$ 2,600		$ 2,600
Tax benefit (expense) related to transition tax			$ 1,300
Tax benefit (expense) related to remeasurement of net deferred tax assets	$ 500
Deferred Tax Assets: State and Local	3,551	616	3,551	616
Decrease in net valuation allowance			800
Unrecognized tax benefits	$ 6,350	$ 0	6,350	$ 0
Interest related to unrecognized tax benefits			860
Interest related to unrecognized tax benefits, net of tax			600
Penalties			$ 0
United Kingdom
Income Taxes [Line Items]
Foreign effective income tax rate			21.00%
Decrease in net valuation allowance			$ 1,200
United Kingdom \| Syndicate 1221 \| Maximum
Income Taxes [Line Items]
Percentage of premiums	50.00%		50.00%
United Kingdom \| Syndicate 1221 \| Minimum
Income Taxes [Line Items]
Foreign effective income tax rate			21.00%