Collateral pledge as security for payments to be received under reinsurance contracts.

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Number of Letters of Credit

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Percentage points added to the reference rate to compute the variable rate on the debt instrument.

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Face (par) amount of debt instrument at time of issuance.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Percentage price of original principal amount of debt at which debt can be redeemed by the issuer.

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Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Including the current and noncurrent portions, the carrying value of junior subordinated long-term debt owed to unconsolidated subsidiary trusts as of the balance sheet date. Such debt places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets and was initially scheduled to be repaid after one year or beyond the normal operating cycle if longer.

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The reporting currency-denominated liability value.

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The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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Long-term Debt - Additional Information (Detail) £ in Millions		12 Months Ended
	Nov. 02, 2018 USD ($)	Dec. 31, 2020 USD ($) debtInstrument	Dec. 31, 2019 USD ($)	Dec. 31, 2019 GBP (£)	Mar. 02, 2017 USD ($)
Debt Instrument [Line Items]
Redemption price, percentage		100.00%
Junior subordinated debentures		$ 257,800,000	$ 257,400,000
Floating rate loan stock denominated in Euros		44,200,000	39,800,000
Principal		143,800,000	143,800,000
Amount of collateral under reinsurance agreement		1,131,400,000	$ 1,184,700,000
Letter of Credit
Debt Instrument [Line Items]
Letter of credit facility amount outstanding		167,000,000.0
New Credit Agreement
Debt Instrument [Line Items]
Line of credit, maximum borrowing amount	$ 325,000,000
New Credit Agreement \| London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Interest rate basis spread			1.25%
Debt instrument interest rate stated percentage			3.18%	3.18%
New Credit Agreement \| Term loan
Debt Instrument [Line Items]
Principal	125,000,000
New Credit Agreement \| Revolving Credit Facility
Debt Instrument [Line Items]
Line of credit, maximum borrowing amount	200,000,000.0
Principal	200,000,000
Letter of credit facility amount outstanding	0
New Credit Agreement \| Letter of Credit
Debt Instrument [Line Items]
Letter of credit facility amount outstanding	70,500,000
Prior Agreement
Debt Instrument [Line Items]
Line of credit, maximum borrowing amount					$ 325,000,000
Repayments of borrowings	$ 125,000,000
Reinsurance Credit Facilities, Secured Bilateral Line of Credit Facilities \| Letter of Credit
Debt Instrument [Line Items]
Letter of credit facility amount outstanding		205,000,000
Amount of collateral under reinsurance agreement		189,400,000
Committed Secured Bilateral Facility \| Letter of Credit
Debt Instrument [Line Items]
Letter of credit facility amount outstanding		$ 58,100,000
Number of letters of credit \| debtInstrument		1
Debt instrument, term		1 year
Uncommitted Secured Bilateral Facility \| Letter of Credit
Debt Instrument [Line Items]
Letter of credit facility amount outstanding		$ 108,900,000
Number of letters of credit \| debtInstrument		2
Funds at Lloyd's Instrument \| Letter of Credit
Debt Instrument [Line Items]
Letter of credit facility amount outstanding \| £				£ 23.3
Unsecured Line of Credit Facility \| Letter of Credit
Debt Instrument [Line Items]
Amount of collateral under reinsurance agreement		$ 3,900,000	$ 3,100,000
Junior subordinated debentures
Debt Instrument [Line Items]
Redemption price, percentage		100.00%	100.00%
Junior Subordinated Debentures, Maybrooke
Debt Instrument [Line Items]
Junior subordinated debentures		$ 91,800,000
Unsecured debentures		85,100,000	$ 84,700,000
Principal		$ 91,800,000	$ 91,800,000
Debt instrument interest rate stated percentage		3.37%	5.04%	5.04%
Junior Subordinated Debentures, Maybrooke \| London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Interest rate basis spread		3.15%	3.15%
Unsecured debt
Debt Instrument [Line Items]
Redemption price, percentage			100.00%