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

No definition available.

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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 Credit Carryforward, Expiration Year

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible state and local operating loss carryforwards.

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

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Amount of deferred tax liability attributable to taxable temporary differences from undistributed earnings of subsidiaries and other recognized entities not within the country of domicile of the entity.

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Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Amount of undistributed earnings of foreign subsidiaries intended to be permanently reinvested outside the country of domicile.

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Amount of unrecognized tax benefits pertaining to uncertain tax positions taken in tax returns.

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Amount of expense for interest on an underpayment of income taxes and penalties related to a tax position claimed or expected to be claimed in the tax return.

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Income Taxes - Additional Information (Detail) (USD $)	3 Months Ended								12 Months Ended			0 Months Ended
Income Taxes - Additional Information (Detail) (USD $)	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jan. 01, 2015
Income Taxes [Line Items]
Domestic effective income tax rate									35.00%	35.00%	35.00%
Undistributed earnings of non-U.S. subsidiaries	$ 22,600,000								$ 22,600,000
Taxes payable on undistributed earnings	2,300,000								2,300,000
Unrecognized tax benefits	0us-gaap_UnrecognizedTaxBenefits				0us-gaap_UnrecognizedTaxBenefits				0us-gaap_UnrecognizedTaxBenefits	0us-gaap_UnrecognizedTaxBenefits
Interest or penalties related to unrecognized tax benefits									0	0
Income tax (benefit) expense	8,823,000us-gaap_IncomeTaxExpenseBenefit	15,032,000us-gaap_IncomeTaxExpenseBenefit	7,998,000us-gaap_IncomeTaxExpenseBenefit	13,354,000us-gaap_IncomeTaxExpenseBenefit	6,076,000us-gaap_IncomeTaxExpenseBenefit	9,804,000us-gaap_IncomeTaxExpenseBenefit	6,284,000us-gaap_IncomeTaxExpenseBenefit	6,643,000us-gaap_IncomeTaxExpenseBenefit	45,207,000us-gaap_IncomeTaxExpenseBenefit	28,807,000us-gaap_IncomeTaxExpenseBenefit	27,974,000us-gaap_IncomeTaxExpenseBenefit
Effective income tax rate									32.20%	31.20%	30.50%
Deferred Tax Assets: State and Local	777,000us-gaap_DeferredTaxAssetsStateTaxes				555,000us-gaap_DeferredTaxAssetsStateTaxes				777,000us-gaap_DeferredTaxAssetsStateTaxes	555,000us-gaap_DeferredTaxAssetsStateTaxes
State and local net operating loss carry-forwards	$ 0				$ 100,000				$ 0	$ 100,000
Syndicate 1221 \| Subsequent Event
Income Taxes [Line Items]
Domestic effective income tax rate												35.00%
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%								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									2032
State and Local Jurisdiction \| Minimum
Income Taxes [Line Items]
State and local tax carry-forwards expiration year									2024