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Credit Facility and Long-Term Debt - Additional Information (Details) - USD ($)				12 Months Ended
	Mar. 27, 2017	Nov. 13, 2013	Dec. 15, 2010	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Nov. 30, 2020	Nov. 25, 2020	Nov. 03, 2020	Feb. 11, 2019	Jun. 30, 2017
Line of Credit Facility [Line Items]
Revolving credit facility, credit agreement maximum limit	$ 200,000,000
Line of credit facility Increasable capacity	$ 100,000,000
Line of credit facility borrowings outstanding				$ 0
Minimum consolidated tangible net worth under credit facility				$ 1,891,000,000
Percentage of consolidated net income				25.00%
Percentage of aggregate net cash proceeds from the issuance of capital stock				25.00%
Percentage of consolidated leverage ratio permitted				35.00%
Payment for extinguishment of debt				$ 0	$ 5,500,000	$ 8,600,000
Repayments of Other Long-term Debt				0	125,000,000.0	$ 125,000,000.0
AUL | Lloyds
Line of Credit Facility [Line Items]
Letter of Credit to Support Funds at Lloyd's									$ 40,000,000
Funds at Lloyd's Facility Agreement				100,000,000			$ 100,000,000	$ 80,000,000
Line of Credit Facility, Maximum Amount Outstanding During Period				80,000,000
Citibank Europe
Line of Credit Facility [Line Items]
Outstanding collateralized letters of credit facility				$ 444,200,000	$ 444,200,000
Letters of credit / guarantees | Aspen U.K. | Aspen European
Line of Credit Facility [Line Items]
Revolving credit facility, credit agreement maximum limit										$ 100,000,000
London Interbank Offered Rate (LIBOR)
Line of Credit Facility [Line Items]
Line of Credit Facility, Interest Rate During Period				1.00%
Citibank Europe plc | Letters of credit / guarantees
Line of Credit Facility [Line Items]
Revolving credit facility, credit agreement maximum limit											$ 500,000,000.0
Senior Notes
Line of Credit Facility [Line Items]
Debt stated interest rate		4.65%	6.00%
Net proceeds from senior notes		$ 299,700,000	$ 247,500,000
Loans issued		$ 300,000,000.0	$ 250,000,000.0
Amount of indebtedness subject to default in payment				$ 50,000,000.0
Annual interest payable				$ 14,000,000.0

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- Definition

Debt default minimum.

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- Definition

Funds at Lloyd's Facility Agreement

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X

- Definition

Letter of Credit to Support Funds at Lloyd's

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No definition available.

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X

- Definition

Line of credit facility Increasable capacity.

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No definition available.

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- Definition

Line of credit facility increase additional borrowing capacity.

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- Definition

Line of credit facility tangible net worth cash proceeds from the issuance of capital stock restriction.

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- Definition

Line of credit facility tangible net worth net income restriction.

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- Definition

Line of credit facility tangible net worth restrictions minimum.

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- Definition

Payment for Extinguishment of Debt

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- Definition

Percentage of consolidated leverage ratio permitted.

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- Definition

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

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- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

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- Definition

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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X

- Definition

The effective interest rate during the reporting period.

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- Definition

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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- Definition

Maximum amount borrowed under the credit facility at any time during the period.

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- Definition

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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- Definition

Amount of the required periodic payment applied to interest.

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- Definition

The cash inflow from a borrowing with the highest claim on the assets of the entity in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle, if longer).

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Amount of cash outflow for the payment of debt classified as other, maturing after one year or the operating cycle, if longer.

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