Aspen Insurance Holdings Limited (AHL-PC) F-1/A 5 Apr 24 Registration statement (foreign) (amended) Financial data

Cover Page

Cover Page	12 Months Ended
	Dec. 31, 2023
Cover [Abstract]
Entity Registrant Name	ASPEN INSURANCE HOLDINGS LIMITED
Entity Emerging Growth Company	false
Amendment Description	The registrant hereby amends this registration statement on such date or dates as may be necessary to delay its effective date until the registrant shall file a further amendment which specifically states that this registration statement shall thereafter become effective in accordance with Section 8(a) of the Securities Act of 1933, as amended, or until the registration statement shall become effective on such date as the Securities and Exchange Commission, acting pursuant to Section 8(a), may determine.
Amendment Flag	true
Entity Central Index Key	0001267395
Document Type	F-1/A

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Investments:
Debt Securities, Available-for-sale	$ 4,122,600	$ 3,788,600
Debt Securities, Trading	1,485,700	1,475,500
Short-term investments, available for sale	93,600	52,000
Short-term investments, trading at fair value	2,100	6,300
Catastrophe bonds, trading at fair value (amortized cost — 2022: $5.1 and 2021: $5.3)	1,600	2,900
Privately-held Investments, available for sale, at fair value	14,900	0
Privately-held Investments	475,000	533,000
Investments, equity method	7,600	6,200
Other investments, at fair value (2)	209,300	221,300
Investments, Excluding Derivative Assets	6,412,400	6,085,800
Reinsurance recoverables:
Cash and cash equivalents (including cash within consolidated variable interest entities of — $65.7448377 and $75.15054353)	1,028,100	959,200
Unpaid losses recoverable from reinsurers (net of allowance for expected credit losses of 2022: $3.7 — 2021: $3.3)	4,577,800	4,897,700
Ceded unearned premiums	733,500	737,300
Receivables:
Underwriting premiums receivables (net of allowance for expected credit losses of 2022: $25.0 — 2021: $30.2)	1,435,300	1,482,400
Deferred acquisition costs	296,200	319,000
Derivatives at fair value	31,700	56,200
Right-of-use operating lease assets	61,600	72,800
Income taxes refundable	4,300	20,800
Deferred tax assets	312,600	120,100
Other assets	309,600	384,200
Intangible assets and goodwill	21,700	21,800
Total assets	15,224,800	15,157,300
Insurance reserves
Losses and loss adjustment expenses	7,810,600	7,710,900
Unearned premiums	2,426,300	2,457,500
Total insurance reserves	10,236,900	10,168,400
Reinsurance premiums	1,416,600	1,980,100
Payables
Income taxes payable	12,600	10,900
Deferred tax liabilities	1,600	900
Accrued expenses and other payables (4)	214,400	201,800
Payables for securities purchased	22,300	6,900
Operating lease liabilities	86,100	95,500
Liabilities under derivative contracts	25,800	34,900
Long-term debt	300,000	0
Short-term debt	0	299,900
Total liabilities	12,316,300	12,799,300
Commitments and contingent liabilities (see Note 20)	0	0
Ordinary shares:
Ordinary shares, value	604	604
Preference shares, value	753,500	753,500
Additional paid-in capital	761,200	761,200
Retained earnings	1,793,500	1,349,000
Accumulated other comprehensive (loss)	(400,300)	(506,300)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	2,908,500	2,358,000
Total liabilities and shareholders’ equity	$ 15,224,800	$ 15,157,300

CONSOLIDATED BALANCE SHEETS Con

CONSOLIDATED BALANCE SHEETS Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Fixed income securities, available for sale amortized cost	$ 4,330	$ 4,131.3
Depositary share interest of 1/1000th in each 5.625%	0.01%
Net allowance, available for sale maturities, for expected credit losses	$ 2.9	7.7
Fixed income securities, trading, amortized cost	1,527	1,576.7
Catastrophe Bonds, Fair Value Disclosure	1.6	5.1
Privately-held Investments, available for sale, at fair value	14.9	0
Privately-held investments, available for sale, amortized cost	14.7	0
Privately-held investments, cost, trading at fair value	494.9	537.7
Premium Receivable, Allowance for Credit Loss	$ 21	$ 25
Ordinary shares, issued	60,395,839	60,395,839
Ordinary shares, par value	$ 0.01	$ 0.01
Unpaid losses recoverable from reinsurers (net of allowance for expected credit losses of 2022: $3.7 — 2021: $3.3)	$ 3.7	$ 3.7
Restricted cash	323.2	232.1
Privately-held Investments	475	533
Other investments, at fair value (2)	209.3	221.3
Cash and cash equivalents, including cash within consolidated VIE		65.7
Accrued expenses and other payables (4)	214.4	201.8
Debt Securities, Trading	1,485.7	1,475.5
Asset Management Arrangement
Accrued expenses and other payables (4)	2.1	4.5
Apollo originating partnership | Fair Value, Recurring [Member]
Privately-held Investments, available for sale, at fair value	14.9	0
Privately-held Investments	112.4	44.8
Other investments, at fair value (2)	15.9	12.7
Apollo real estate fund | Fair Value, Recurring [Member]
Other investments, at fair value (2)	23.9	25.3
Management Consulting Agreement
Accrued expenses and other payables (4)	1.2	1.3
CLOs | Fair Value, Recurring [Member]
Debt Securities, Trading	129.8	0
Fixed maturities
Fixed income securities, available for sale amortized cost	4,438.3	4,183.7
Net allowance, available for sale maturities, for expected credit losses	2.9	7.7
Short-term investments
Fixed income securities, trading, amortized cost	$ 2.1	$ 6.3

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS AND OTHER COMPREHENSIVE INCOME - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Revenues
Net earned premiums	$ 2,614.5	$ 2,688.7	$ 2,410.5
Net investment income (1)	275.7	188.1	147.5
Realized and unrealized investment gains	75.9	5	56.2
Other income	0	8.2	14.7
Total revenues	2,966.1	2,890	2,628.9
Expenses
Losses and loss adjustment expenses	1,553	1,680	1,693.3
Acquisition costs	380.2	431.8	414.1
General, administrative and corporate expenses (2)	503.6	494.2	418
Interest expense	55.2	43.7	14.3
Change in fair value of derivatives	(26.1)	80.5	35.9
Realized and unrealized investment losses (3)	61.4	182.6	47.4
Net realized and unrealized foreign exchange (gains)/losses	(36.2)	15.9	40
Other expenses	0	20.1	10.8
Total expenses	2,563.5	2,917	2,593.8
Income (loss) from operations before income taxes	402.6	(27)	35.1
Income tax benefit/(expense)	132.1	78.1	(5.3)
Net income/(loss) attributable to Aspen Insurance Holdings Limited’s ordinary shareholders	534.7	51.1	29.8
Available for sale investments:
Reclassification adjustment for net realized gains/(losses) on investments included in net income/(loss)	40.2	55.5	(20.4)
Change in net unrealized (losses)/gains on available for sale securities held	86	(447.2)	(137.2)
Net change from current period hedged transactions	(14)	15.4	(6.2)
Change in foreign currency translation adjustment	14.4	(30.9)	21.4
Other comprehensive (loss)/income, before income taxes	126.6	(407.2)	(142.4)
Income tax benefit/(expense) thereon:
Reclassification adjustment for net realized losses on investments included in net /(loss)	6.6	0	0
Change in net unrealized gains on available for sale securities held	(14)	23.9	(0.3)
Total income tax benefit/(expense) allocated to other comprehensive (loss)	(20.6)	23.9	(0.3)
Other comprehensive (loss)/income, net of tax	106	(383.3)	(142.7)
Total comprehensive (loss)/income attributable to Aspen Insurance Holdings Limited's ordinary shareholders	640.7	(332.2)	(112.9)
Net income
Net (loss)/income attributable to Aspen Insurance Holdings Limited’s ordinary shareholders	534.7	51.1	29.8
Preference share dividends	49.9	44.6	44.5
Net income/(loss) available to Aspen Insurance Holdings Limited’s ordinary shareholders	$ 484.8	$ 6.5	$ (14.7)

Income (Parenthetical)

Income (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Net investment income (1)	$ 275.7	$ 188.1	$ 147.5
General and administrative expenses	354.5	386.5	333.1
Realized and unrealized investment losses	61.4	182.6	47.4
Asset Management Arrangement
Net investment income (1)	9.4	4.9	5.8
Management Consulting Agreement
General and administrative expenses	5	5	5
Apollo, Class A & B Notes
Realized and unrealized investment losses	8.7	(0.4)	0
Management Consulting Agreement
Net investment income (1)	$ (19.6)	$ (3.1)	$ 0

CONSOLIDATED STATEMENTS OF CHAN

CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY - USD ($)	Total	Ordinary shares	Preference shares	Additional paid-in capital	Retained earnings	Retained earnings Ordinary shares	Cumulative foreign currency translation adjustments:	Unrealized appreciation on available for sale investments:	Gain on derivatives, net of taxes
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest		$ 600,000	$ 753,500,000	$ 716,200,000	$ 1,397,200,000		$ (177,400,000)	$ 192,500,000	$ 4,600,000
Capital contribution	$ 45,000,000
Net income/(loss) for the year	29,800,000
Dividends						$ 0
Change for the year, net of income taxes							21,400,000	(157,900,000)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	2,774,800,000	600,000	753,500,000	761,200,000	1,382,500,000		(156,000,000)	34,600,000	(1,600,000)
Net change from current period hedged transactions									(6,200,000)
Total accumulated other comprehensive (loss)/income	(123,000,000)
Capital contribution	0
Net income/(loss) for the year	51,100,000
Dividends						(40,000,000)
Change for the year, net of income taxes							(30,900,000)	(367,800,000)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	2,358,000,000	600,000	753,500,000	761,200,000	1,349,000,000		(186,900,000)	(333,200,000)	13,800,000
Net change from current period hedged transactions									15,400,000
Total accumulated other comprehensive (loss)/income	(506,300,000)
Aggregate liquidation preferences	775,000,000
Capital contribution	0
Net income/(loss) for the year	534,700,000
Dividends	(49,865,700)					$ (40,300,000)
Change for the year, net of income taxes							14,400,000	105,600,000
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	2,908,500,000	$ 600,000	$ 753,500,000	$ 761,200,000	$ 1,793,500,000		$ (172,500,000)	$ (227,600,000)	(200,000)
Net change from current period hedged transactions									$ (14,000,000)
Total accumulated other comprehensive (loss)/income	(400,300,000)
Aggregate liquidation preferences	$ 775,000,000

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Cash flows (used in)/from operating activities:
Net income/(loss)	$ 534,700,000	$ 51,100,000	$ 29,800,000
Adjustments to reconcile net income/(loss) to net cash flows from operating activities:
Depreciation and amortization	11,000,000	43,300,000	53,300,000
Impairment of lease assets	0	(6,700,000)	400,000
Amortization of right-of-use operating lease assets	10,700,000	10,100,000	12,000,000
Interest on operating lease liabilities	4,500,000	5,400,000	5,500,000
Realized and unrealized investment gains	(75,900,000)	(5,000,000)	(56,200,000)
Realized and unrealized investment losses	61,400,000	182,600,000	47,400,000
Deferred tax expense/(benefit)	(197,700,000)	(104,600,000)	(3,200,000)
Net realized and unrealized investment foreign exchange (gains)/losses	(5,300,000)	15,900,000	13,000,000
Net change from current period hedged transactions	(14,000,000)	15,400,000	(6,200,000)
Unrealized (gain)/loss on real estate fund in net investment income	17,900,000	(14,500,000)	(20,500,000)
Insurance reserves:
Losses and loss adjustment expenses	99,700,000	99,100,000	483,300,000
Unearned premiums	(31,200,000)	345,200,000	310,200,000
Reinsurance recoverables:
Unpaid losses	(319,900,000)	1,599,600,000	109,500,000
Ceded unearned premiums	3,800,000	(141,200,000)	(143,100,000)
Other receivables	11,700,000	(26,400,000)	(19,400,000)
Deferred acquisition costs	22,800,000	(28,200,000)	13,200,000
Reinsurance premiums payable	(563,500,000)	1,404,400,000	1,200,000
Funds withheld	43,600,000	1,100,000	(5,700,000)
Premiums receivable	47,100,000	(177,800,000)	(142,900,000)
Income tax payable	3,500,000	(1,300,000)	(7,600,000)
Accrued expenses and other payable	13,200,000	(87,000,000)	73,500,000
Fair value of derivatives and settlement of liabilities under derivatives	15,400,000	(21,700,000)	13,600,000
Operating lease liabilities	(15,500,000)	(15,500,000)	(17,500,000)
Other assets	6,900,000	900,000	100,000
Net cash (used in)/from operating activities	324,700,000	(55,000,000)	524,700,000
Cash flows (used in)/from investing activities:
(Purchases) of fixed income securities — Available for sale	(1,554,800,000)	(1,613,900,000)	(2,217,300,000)
(Purchases) of fixed income securities — Trading	(418,500,000)	(724,600,000)	(866,400,000)
Proceeds from sales and maturities of fixed income securities — Available for sale	1,326,700,000	2,212,500,000	1,538,100,000
Proceeds from sales and maturities of fixed income securities — Trading	474,000,000	293,600,000	548,200,000
Net proceeds from catastrophe bonds — Trading	1,500,000	500,000	14,300,000
(Purchases) of short-term investments — Available for sale	(265,900,000)	(55,900,000)	(17,200,000)
Proceeds from sale of short-term investments — Available for sale	231,000,000	13,600,000	99,600,000
(Purchases) of short-term investments — Trading	(15,100,000)	(7,000,000)	(26,800,000)
Proceeds from sale of short-term investments — Trading	19,500,000	2,600,000	60,800,000
(Purchases) of privately-held investments - Available for sale	(14,700,000)	0	0
(Purchases) of privately-held investments — Trading	(99,000,000)	(377,900,000)	(205,100,000)
Proceeds from sale of privately-held investments — Trading	136,900,000	147,400,000	182,100,000
Net change in (payable)/receivable for securities (purchased)/sold	19,900,000	(31,800,000)	26,600,000
(Purchases) of other investments	(9,300,000)	(62,500,000)	(20,000,000)
Net proceeds from sales of other investments	4,900,000	5,900,000	0
Net (purchases)/sales of fixed assets	8,900,000	(3,000,000)	64,500,000
Net (purchases)/sales of investments, equity method	(400,000)	(2,000,000)	(2,700,000)
Net cash (used in)/from investing activities	(172,200,000)	(196,500,000)	(950,300,000)
Cash flows used in/from financing activities:
Repayments of Short-Term Debt	(300,000,000)	0	0
Proceeds from term loan facility	300,000,000	0	0
Capital contribution	0	0	45,000,000
Dividends paid on ordinary shares	40,300,000	40,000,000	0
Dividends paid on preference shares	(49,900,000)	(44,600,000)	(44,500,000)
Net cash (used in)/from financing activities	(90,200,000)	(84,600,000)	500,000
Effect of exchange rate movements on cash and cash equivalents	6,600,000	(18,800,000)	(8,100,000)
(Decrease)/increase in cash and cash equivalents	68,900,000	(354,900,000)	(433,200,000)
Cash and cash equivalents at beginning of period	959,200,000	1,314,100,000	1,747,300,000
Cash and cash equivalents at end of period (1)	1,028,100,000	959,200,000	1,314,100,000
Restricted cash	323,200,000	232,100,000	364,900,000
Supplemental disclosure of cash flow information:
Net cash paid during the period for income taxes	60,900,000	29,100,000	15,700,000
Cash paid during the period for interest	$ 15,600,000	$ 14,300,000	$ 14,000,000

History and Organization

History and Organization	12 Months Ended
	Dec. 31, 2023
Accounting Policies [Abstract]
History and Organization	History and Organization History and Organization. Aspen Insurance Holdings Limited (“Aspen Holdings”) was incorporated on May 23, 2002 as a holding company headquartered in Bermuda. We underwrite specialty insurance and reinsurance on a global basis through our Operating Subsidiaries (as defined below) based in Bermuda, the United States and the United Kingdom: Aspen Bermuda Limited (“Aspen Bermuda”), Aspen Specialty Insurance Company (“Aspen Specialty”), Aspen American Insurance Company (“AAIC”), Aspen Insurance UK Limited (“Aspen UK”) and Aspen Underwriting Limited (“AUL”) (as corporate member of our Lloyd’s operations, Syndicate 4711, which are managed by Aspen Managing Agency Limited (“AMAL”) (together, “Aspen Lloyd’s”)), each referred to herein as an “Operating Subsidiary” and collectively referred to as the “Operating Subsidiaries”. We also have branches in Australia, Canada, Singapore and Switzerland. We established Aspen Capital Management, Ltd. (“ACML”) and other related entities (collectively, “ACM”) to leverage our existing underwriting franchise, increase our operational flexibility and provide third-party investors direct access to our capital markets and underwriting expertise. References to the “Company,” the “Group,” “we,” “us” or “our” refer to Aspen Holdings or Aspen Holdings and its consolidated subsidiaries. Since February 2019, the Company has been a wholly-owned subsidiary of Highlands Bermuda Holdco, Ltd. (“Parent”), which holds all of the Company’s ordinary shares. Parent, a Bermuda exempted company, is an affiliate of certain investment funds managed by affiliates of Apollo Global Management, Inc., a leading global investment manager (collectively with its subsidiaries, “Apollo”). The Company’s preference shares and depositary shares are listed on the New York Stock Exchange (“NYSE”) under the following symbols: AHL PRC, AHL PRD and AHL PRE.

Basis of Preparation and Signif

Basis of Preparation and Significant Accounting Policies	12 Months Ended
	Dec. 31, 2023
Accounting Policies [Abstract]
Basis of Preparation and Significant Accounting Policies	Basis of Presentation and Significant Accounting Policies The consolidated financial statements of Aspen Holdings are prepared in accordance with United States Generally Accepted Accounting Principles (“U.S. GAAP”) and are presented on a consolidated basis including the transactions of all operating subsidiaries in which the Company has a controlling financial interest and variable interest entities (“VIE”) in which the Company is considered to be the primary beneficiary. Transactions between Aspen Holdings and its subsidiaries are eliminated within the consolidated financial statements. The consolidated financial statements have been prepared on a going concern basis. To facilitate comparison of information across periods, certain reclassifications have been made to prior year amounts to conform to the current year’s presentation. (a) Use of Estimates Assumptions and estimates made by management have a significant effect on the amounts reported within the consolidated financial statements. The most significant of these relate to losses and loss adjustment expenses, reinsurance recoverables, gross written premiums and commissions which have not been reported to the Company such as those relating to proportional treaty reinsurance contracts, unrecognized tax benefits, recoverability of deferred tax assets, the fair value of derivatives and the fair value of other and privately-held investments. All material assumptions and estimates are regularly reviewed and adjustments made as necessary but actual results could be significantly different from those expected when the assumptions or estimates were made. (b) Accounting for Insurance and Reinsurance Operations Premiums Earned. Premiums are generally recorded as written on the inception date of a policy. For proportional reinsurance treaty contracts, written premiums are generally recorded as the reinsured policies attach to the treaty. For multi-year insurance or reinsurance contracts, written premiums are recorded based on the contract terms. Premiums are recognized as revenues proportionately over the coverage period. Premiums earned are recorded in the consolidated statements of operations, net of the cost of purchased reinsurance. Premiums written which are not yet recognized as earned premium are recorded in the consolidated balance sheet as unearned premiums. Written and earned premiums and the related costs include estimates for premiums which have not been finally determined. These relate mainly to contractual provisions for the payment of adjustment or additional premiums, premiums payable under proportional treaties and delegated underwriting authorities, and reinstatement premiums. Adjustment and additional premiums are premiums charged which relate to experience during the policy term. The proportion of adjustable premiums included in the premium estimates varies between business lines with the largest adjustment premiums being in property and casualty reinsurance, marine, aviation and energy insurance and the smallest in property and casualty insurance. Premiums under proportional treaty contracts and delegated underwriting authorities are generally not reported to the Company until after the reinsurance coverage is in force. As a result, an estimate of these “pipeline” premiums is recorded. The Company estimates pipeline premiums based on projections of ultimate premium taking into account reported premiums and expected development patterns. Reinstatement premiums on assumed excess of loss reinsurance contracts are provided based on experience under such contracts. Reinstatement premiums are the premiums charged for the restoration of the reinsurance limit of an excess of loss contract to its full amount after payment by the reinsurer of losses as a result of an occurrence. The original premiums are recognized as revenue in full at the date of loss, with the reinstatement premiums recognized as revenue over the remaining cover term. Reinstatement premiums provide future insurance cover for the remainder of the initial policy term. An allowance for uncollectible premiums is established for possible non-payment of premium receivables, as deemed necessary. Credit Losses on Underwriting Premiums Receivable. Underwriting premium receivable balances are reported net of an allowance for expected credit losses. The allowance, based on ongoing review and monitoring of amounts outstanding, historical loss data, including write-offs and other current economic factors, is charged to net income in the period the receivable is recorded and revised in subsequent periods to reflect changes in the Company’s estimate of expected credit losses. For most insurance policies, credit risk is partially mitigated by the Company’s ability to cancel the policy if the policyholder does not pay the premium whereby, upon default, policy liabilities would be written-down along with premium receivables. Losses and Loss Adjustment Expenses. Losses represent the amount paid or expected to be paid to claimants in respect of events that have occurred on or before the balance sheet date. The costs of investigating, resolving and processing these claims are known as loss adjustment expenses (“LAE”). The consolidated statements of operations records these losses net of reinsurance, meaning that gross losses and loss adjustment expenses incurred are reduced by the amounts recovered or expected to be recovered under reinsurance contracts. Reinsurance. Written premiums, earned premiums, incurred claims, LAE and the acquisition costs all reflect the net effect of assumed and ceded reinsurance transactions. Assumed reinsurance refers to the Company’s acceptance of certain insurance risks that other insurance companies have underwritten. Ceded reinsurance arises from contracts under which other insurance companies agree to share certain risks with the Company. Reinsurance accounting is followed when there is significant timing risk, underwriting risk and transfer risk, and a reasonable possibility of significant loss. Outward reinsurance premiums, which are paid when the Company purchases reinsurance or retrocessional coverage, are accounted for using the same accounting methodology as the Company uses for inwards premiums. Premiums payable under reinsurance contracts that operate on a “losses occurring during” basis are expensed over the period of coverage while those arising from “risks attaching during” policies are expensed over the earnings period of the underlying premiums written from the reinsured business. Adjustment premiums and reinstatement premiums in relation to outward reinsurance are accrued when it is determined that the ultimate losses will trigger a payment and recognized within premiums payable. Reinsurance and retrocession does not isolate the ceding company from its obligations to policyholders. In the event that a reinsurer or retrocessionaire fails to meet its obligations, the ceding company’s obligations remain. Accounting for Retroactive Reinsurance Agreements. Retroactive reinsurance agreements are reinsurance agreements under which a reinsurer agrees to reimburse the Company as a result of past insurable events. For retroactive reinsurance purchased by the Company, the excess of the amounts ultimately collectible under the agreement over the consideration paid is recognized as a deferred gain liability which is amortized into income over the settlement period of the ceded reserves once the paid losses have exceeded the minimum retention. The amount of the deferral is recalculated each period based on actual loss payments and updated estimates of ultimate losses. If the consideration paid exceeds the ultimate losses collectible under the agreement, the net loss on the retroactive reinsurance agreement is recognized within income immediately. Premiums payable for retroactive reinsurance coverage and meeting the conditions of reinsurance accounting are reported as reinsurance recoverables to the extent that those amounts do not exceed recorded liabilities relating to underlying reinsurance contracts. Premiums paid in excess of accounts receivable are charged to income. Reserves. Insurance reserves are established for the total unpaid cost of claims and LAE in respect of events that have occurred by the balance sheet date, including the Company’s estimates of the total cost of claims incurred but not yet reported (“IBNR”). Claim reserves are reduced for estimated amounts of salvage and subrogation recoveries. Estimated amounts recoverable from reinsurers on unpaid losses and LAE are reflected as assets. For reported claims, reserves are established on a case-by-case basis within the parameters of coverage provided in the insurance policy or reinsurance agreement. For IBNR claims, reserves are estimated using a number of established actuarial methods to establish a range of estimates from which a management best estimate is selected. Both case and IBNR reserve estimates consider variables such as past loss experience, changes in legislative conditions, changes in judicial interpretation of legal liability, policy coverages and inflation. As many of the coverages underwritten involve claims that may not be ultimately settled for many years after they are incurred, subjective judgments as to the ultimate exposure to losses are an integral and necessary component of the loss reserving process. The Company regularly reviews its reserves, using a variety of statistical and actuarial techniques to analyze current claims costs, frequency and severity data, and prevailing economic, social and legal factors. Reserves established in prior periods are adjusted as claim experience develops and new information becomes available. Adjustments to previously estimated reserves are reflected in the financial results of the period in which the adjustments are made. The process of estimating required reserves does, by its very nature, involve considerable uncertainty. The level of uncertainty can be influenced by factors such as the existence of coverage with long duration payment patterns and changes in claims handling practices, as well as the factors noted above. Ultimate actual payments for claims and LAE could turn out to be significantly different from the Company’s estimates. Credit Losses on Reinsurance Recoverables. Amounts recoverable from reinsurers are estimated in a manner consistent with the claim liability with the reinsured business. The Company maintains credit risk if a reinsurer is unable to pay recoverables when they become due. To manage this risk, the Company evaluates the financial condition of its reinsurers and retrocessionaires, and monitors concentration of credit risk to minimize its exposure to significant losses from individual reinsurers. To further reduce credit exposure on reinsurance recoverables, the Company has received collateral, including letters of credit and trust accounts, from certain reinsurers. Following the adoption of ASC 326, an allowance is established for expected credit losses to be recognized over the life of the reinsurance recoverable. The allowance considers the current financial strength of the individual reinsurer and the amount of collateral held. Acquisition Costs. The costs directly related to writing a (re)insurance policy are referred to as acquisition expenses and include commissions, premium taxes and profit commissions. With the exception of profit commissions, these expenses are incurred when a policy is issued, and only the costs directly related to the successful acquisition of new and renewal insurance and reinsurance contracts are deferred and amortized over the same period as the corresponding premiums are recorded as revenues. Profit commissions are estimated and accrued based on the related performance criteria evaluated at the balance sheet date, with subsequent changes to those estimates recognized when they occur. Commissions received related to reinsurance premiums ceded are netted against broker commissions in determining acquisition costs eligible for deferral. On a regular basis a premium deficiency analysis is performed of the deferred acquisition costs in relation to the expected recognition of revenues, including anticipated investment income, and adjustments, if any, are reflected as period costs. Should the analysis indicate that the acquisition costs are unrecoverable, further analyses are performed to determine if a reserve is required to provide for losses which may exceed the related unearned premium. General and Administrative Expenses. These costs represent the expenses incurred in running the business and include, but are not limited to compensation costs for employees, rental costs, IT development and professional and consultancy fees. General and administrative costs directly attributable to the successful acquisition of business are deferred and amortized over the same period as the corresponding premiums are recorded as revenues. When reporting the results for its business segments, the Company includes expenses which are directly attributable to the segment plus an allocation of central administrative costs. Corporate Expenses. Corporate expenses are not allocated to the Company’s business segments as they typically do not fluctuate with the levels of premium written and are related to the Company’s operations which include group executive costs, group finance costs, group legal and actuarial costs and certain strategic and other costs. (c) Accounting for Investments, Cash and Cash Equivalents Fixed Income Securities. The fixed income securities portfolio comprises securities issued by governments and government agencies, corporate bonds, mortgage and other asset-backed securities and bank loans. Investments in fixed income securities are classified as available for sale or trading and are reported at estimated fair value in the consolidated balance sheet. Investment transactions are recorded on the trade date with balances pending settlement reflected in the consolidated balance sheet under receivables for securities sold and other payables for securities purchased, respectively. Fair values are based on quoted market prices and other data provided by third-party pricing services. Short-term Investments. Short-term investments primarily comprise highly liquid debt securities with a maturity greater than three months but less than one year from the date of purchase and are held as part of the investment portfolio of the Company. Short-term investments are classified as either trading or available for sale and reported at estimated fair value. Catastrophe Bonds. Investments in catastrophe bonds are classified as trading and are reported on the consolidated balance sheet at estimated fair value. The fair values are based on independent broker-dealer quotes. Privately-held Investments. The Company’s privately-held investments primarily comprise commercial mortgage loans, middle market loans and other private debt, asset-backed securities and global corporate securities. These investments are classified as trading or available for sale and are reported on the consolidated balance sheet at estimated fair value. Privately-held investments are initially valued at cost or transaction value which approximates fair value. In subsequent measurement periods, the fair values of these securities are primarily determined using discounted cash flow models. Interest income is accrued on the principal amount of the loan based on its contractual interest rate subject to it being probable that we will receive interest on that particular underlying loan. Interest income, amortization of premiums and discounts, and prepayment fees are reported in net investment income on the consolidated statements of income. Other Investments, Equity Method. Other investments represent the Company’s investments that are recorded using the equity method of accounting. Adjustments to the fair value of these investments are made based on the net asset value of the investment. Other Investments. Other investments represent the Company’s investments in investment funds that are reported at fair value. Adjustments to the fair value are made based on the net asset value of the investment. Cash and Cash Equivalents. Cash and cash equivalents are reported at fair value. Cash and cash equivalents comprise cash on hand, deposits held on call with banks and other short-term highly liquid investments due to mature within three months from the date of purchase and which are subject to insignificant risk of change in fair value. Gains and Losses. Realized gains or losses on the sale of investments are determined on the basis of the first in first out cost method and are recorded in revenue or expenses respectively. Unrealized gains and losses represent the difference between the cost, or the cost as adjusted by amortization of any difference between its cost and its redemption value (“amortized cost”), of the security and its fair value at the reporting date and are included within other comprehensive income for securities classified as available for sale and in realized and unrealized investment gains or losses in the consolidated statement of operations for securities classified as trading. Credit Losses on Available for Sale Debt Securities . An allowance account for credit losses is recognized for available for sale debt securities based on a review of individual securities. Write-offs are recorded when amounts are deemed uncollectible, or Aspen intends to sell (or more likely than not will be required to sell) the debt security before recovery of the amortized cost basis. The amortized cost basis will be written down to the debt securities fair value through earnings. Credit losses are limited to the difference between the debt securities amortized cost basis and fair value (‘fair-value floor’). Any decline in the debt securities fair value below the amortized cost basis that is not a result of a credit loss is recorded through other comprehensive income, net of applicable taxes. The allowance for credit losses of a security may be increased or reversed upon a change in credit position with the change reflected in net income. The credit loss models employ a discounted cash flow approach to evaluate whether a credit loss exists at the individual security level and are reviewed at each reporting period. This analysis excludes investments in U.S. Government / Agency bonds and U.S. Government Agency mortgage-backed securities due to being of ‘high credit quality’ based on the absence of risk. For any available for sale debt securities that were initially purchased with credit deterioration (PCD), the amortized cost basis shall be considered to be the purchase price, plus any allowance for credit losses. Estimated credit losses shall be discounted at the rate that equates the present value of the purchaser’s estimate of the security’s future cash flows with the purchase price of the asset. Net Investment Income. Investment income includes amounts received and accrued in respect of periodic interest (“coupons”) payable to the Company by the issuer of fixed income securities, equity dividends and interest credited on cash and cash equivalents. It also includes amortization of premium and accretion of discount in respect of fixed income securities. Investment income also includes changes in fair value from investments in real estate funds. Investment management and custody fees are charged against net investment income reported in the consolidated statement of operations. (d) Accounting for Derivative Financial Instruments The Company enters into derivative instruments to manage certain market risks, such as forward exchange contracts used to reduce foreign currency risk relative to the U.S. dollar. The Company records derivative instruments at fair value on the Company’s consolidated balance sheet as either assets or liabilities, depending on their rights and obligations. The accounting for the gain or loss due to the changes in the fair value of these instruments is dependent on whether the derivative qualifies as a hedge. If the derivative does not qualify as a hedge, the gains or losses are reported in the consolidated statement of operations when they occur and classified within Change in fair value of derivatives . If the derivative qualifies as a hedge, the accounting treatment varies based on the type of risk being hedged. There are two primary types of hedging relationships that may be used for accounting purposes: fair value hedge and cash flow hedge. A fair value hedge is designed to offset changes in the fair value of an underlying asset or liability, and the gain or loss from the hedging instrument offsets the change in fair value of the underlying asset or liability. Under fair value hedge accounting, both the gain or loss from the underlying asset or liability and the gain or loss from the hedging instrument are recognized in earnings in the same period. In contrast, a cash flow hedge is designed to offset changes in cash flows of an underlying asset or liability. The gain or loss from the hedging instrument is initially recognized in other comprehensive income. As the contracts settle, the realized gain or loss is reclassified from other comprehensive income into the consolidated statement of operations. The loss portfolio transfer contract includes a funds withheld arrangement that provides a variable interest expense based on Aspen’s investment performance. As a result, this funds withheld arrangement is considered an embedded derivative and accounted for as an option-based derivative. Since the economic characteristics and risks of an embedded derivative feature are not clearly and closely related to the economic characteristics and risks of the host contract, the embedded derivative is bifurcated and accounted for separately at fair value. The Company records subsequent changes in the embedded derivative fair value in the consolidated statement of operations. (e) Accounting for Intangible Assets Intangible assets are held in the consolidated balance sheet at cost less amortization and impairment. Amortization applies on a straight-line basis in respect of assets having a finite estimated useful economic life. Finite intangibles are assessed on an annual basis for impairment, or more frequently where circumstances indicate the carrying value may not be recoverable. For intangible assets considered to have an indefinite life, the Company performs a qualitative assessment annually to determine whether it is more likely than not that an intangible asset is impaired. Goodwill is assessed annually for impairment or more frequently if circumstances indicate an impairment may have occurred. (f) Accounting for Office Properties and Equipment Office properties and equipment are reported at cost less accumulated depreciation. These assets are depreciated on a straight-line basis over the estimated useful lives of the assets. Computer equipment is depreciated between three IT development costs that are directly associated with the development of identifiable and unique software products and that are anticipated to generate economic benefits exceeding costs beyond one year, are recognized within office properties and equipment. Costs include external consultants’ fees, certain qualifying internal staff costs and other costs incurred to develop software programs. Software is depreciated over their estimated useful life, between three (g) Accounting for Leases In the ordinary course of the business, the Company renews and enters into new leases for office real estate and other assets. At the lease inception date, the Company determines whether a contract contains a lease and recognizes operating lease Right-of-use assets and operating lease liabilities based on the present value of future minimum lease payments. As our leases do not provide an implicit interest rate, we use our incremental borrowing rate based on the information available at the commencement date to determine the present value of future payments. For all office real estate leases, rent incentives, including reduced-rent and rent free periods and contractually agreed rent increases during the lease term, have been included when determining the present value of future cash flows. Right-of-use operating lease assets are reported at cost less accumulated depreciation on the consolidated balance sheet and depreciated over the lease term. The Company does not record office property and equipment leases with an initial term of 12 months or less (short-term) in the Company's consolidated balance sheets. Such short-term leases are expensed through the consolidated statement of operations. Right-of-use operating lease assets are tested for impairments whenever events or changes in circumstances indicate that the carrying amount may not be recoverable. If the carrying value of an asset is impaired, it is reduced to the recoverable amount by an immediate charge to the income statement. The recoverable amount is the higher of an asset’s fair value less costs to sell and value in use. (h) Accounting for Foreign Currencies Translation The functional currency of the Company and its subsidiaries is the U.S. Dollar, which is also the Company’s reporting currency. Transactions in currencies other than the functional currency are measured at the exchange rate prevailing at the date of the transaction. Monetary assets and liabilities denominated in non-functional currencies are remeasured at the exchange rate prevailing at the balance sheet date and any resulting foreign exchange gains or losses are reflected in the consolidated statement of operations. Foreign exchange gains or losses related to available for sale investments denominated in non-functional currencies are included within other comprehensive income. Non-monetary assets and liabilities are remeasured to functional currency at historic exchange rates. (i) Accounting for Income Taxes Income taxes are accounted for under the asset and liability method. Deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases and for operating loss and tax credit carryforwards. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. When the Company does not believe that, on the basis of available information, it is more likely than not that deferred tax assets will be fully recovered, it recognizes a valuation allowance against its deferred tax assets to reduce the deferred tax assets to the amount more likely than not to be realized. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that includes the enactment date. Furthermore, a tax benefit from a tax position may be recognized in the financial statements only if it is more-likely-than-not that the position is sustainable, based solely on its technical merits and consideration of the relevant tax authority’s widely understood administrative practices and precedents. The Company applies a portfolio approach to release the income tax effects in accumulated other comprehensive income. Under this approach, the income tax effects upon the sale of an available for sale debt security, settlement of hedged transactions and upon foreign currency translation adjustments for each period, are determined under the intraperiod tax allocation approach. Any tax effects remaining in accumulated other comprehensive income are only released when the entire portfolio is liquidated, sold or extinguished. (j) Accounting for Preference Shares The Company had at the balance sheet date in issue three classes of preference shares. The Company has no obligation to pay interest on these securities but they carry entitlements to dividends payable at the discretion of the Board of Directors. In the event of non-payment of dividends for six consecutive periods, holders of preference shares have director appointment rights. The preference shares are therefore accounted for as equity instruments and included within total shareholders’ equity. (k) Accounting for Share-Based Payments and Long-Term Incentive Plans The Company operates an employee long-term incentive plan, comprised of Performance Units and Exit Units, the terms and conditions of which are described in Note 17. The Company applies a fair-value based measurement method in calculating the compensation costs of Performance Units which are recognized on a straight-line basis over the vesting period. Certain employees of the Company participate in a Management Equity Plan (“MEP”), comprising stock options to acquire non-voting shares in a related party affiliate of the Company, at no cost to the employee. The terms and conditions of the MEP are described in Note 17. The Company recognizes compensation costs for these awards with performance conditions if, and when, the Company concludes that it is probable that the performance condition(s) will be achieved, over the requisite service period. The Company reviews and evaluates these estimates regularly and will recognise any remaining unrecognised compensation expense as an estimate revision. The stock options vest upon a number of performance conditions; an exit or liquidity event occurring; a two-year cumulative operating income hurdle being achieved over the vesting period; and certain other contractual terms being achieved. The Company applies fair-value based measurement principles to determine grant date values for the stock options. The Company has made an entity-wide accounting election to account for award forfeitures as they occur. (l) Accounting for Business Combinations The Company accounts for a transaction as a business combination where the assets acquired and liabilities assumed following a transaction constitute a business. An acquired entity must have inputs and processes that make it capable of generating a return or economic benefit to be considered a business. If the assets acquired are not a business, the Company accounts the transaction as an asset acquisition. The Company recognizes and measures at fair value 100 percent of the assets and liabilities of any acquired business. Goodwill is recognized and measured as the difference between the consideration paid or payable less the fair value of assets acquired. The Company accounts for the disposal of subsidiary undertakings when it ceases to control the subsidiary’s assets and liabilities or the group of assets. A gain or loss is recognized and measured as the difference between the fair value of consideration received or receivable and the value of assets, liabilities and equity components de-recognized, related to that subsidiary or group of assets when deconsolidated. Costs that are directly related to a business combination transaction are expensed in the periods in which the costs are incurred and the services are received. (m) Accounting Pronouncements Accounting Pronouncements Not Yet Adopted Segment Reporting In November 2023, the FASB issued Accounting Standards Update (“ASU”) No. 2023-07, “Segment Reporting (Topic 280) - Improvements to Reportable Segment Disclosures” . This update improves the disclosures about a public entity’s reportable segments and addresses requests from investors for additional, more detailed information about a reportable segment’s expenses. The amendments in this ASU are effective for fiscal years beginning after December 15, 2023, and interim periods within fiscal years beginning after December 15, 2024. As this guidance relates solely to financial statement disclosures, the adoption of ASU 2023-07 will have no impact upon the Company’s results of operations, financial condition, or liquidity. Income Taxes In December 2023, the FASB issued ASU 2023-09, “Income Taxes (Topic 740) - Improvements to Income Tax Disclosures” . The amendments in this Update provide additional transparency about income tax information through improvements to income tax disclosures primarily related to the rate reconciliation and income taxes paid information. The amendments to this Update are effective for annual periods beginning after December 15, 2024. As this guidance relates solely to financial statement disclosures, the adoption of ASU 2023-09 will have no impact upon the Company’s results of operations, financial condition, or liquidity. Other accounting pronouncements were issued during

Segment Reporting

Segment Reporting	12 Months Ended
	Dec. 31, 2023
Segment Reporting [Abstract]
Segment Reporting	Segment Reporting The Company manages its underwriting operations as two business segments: Aspen Reinsurance and Aspen Insurance. The Company has determined its reportable segments by taking into account the manner in which management makes operating decisions and assesses operating performance. Profit or loss for each of the Company’s business segments is measured by underwriting income or loss. Underwriting profit is the excess of net earned premiums over the sum of losses and loss expenses, acquisition costs and general and administrative expenses. Underwriting income or loss provides a basis for management to evaluate the segment’s underwriting performance. Reinsurance Segment. The reinsurance segment consists of property catastrophe reinsurance, other property reinsurance, casualty reinsurance and specialty reinsurance. Insurance Segment. The insurance segment consists of first party insurance, specialty insurance, casualty and liability insurance, and financial and professional lines insurance. Additionally, the insurance segment includes Aspen Underwriting Limited’s participation as a corporate member in Carbon Syndicate 4747 (“Carbon Syndicate”). Non-underwriting Disclosures. The Company provides additional disclosures for corporate and other (non-operating) income and expenses. Corporate and other income and expenses include: corporate expenses, non- operating expenses, net investment income, net realized and unrealized investment gains or losses, changes in fair value of derivatives, interest expenses, net realized and unrealized foreign exchange gains or losses, and income taxes. These income and expense items are not allocated to the Company’s business segments as they are not directly related to the Company’s business segment operations and is consistent with how management measures the performance of its segments. The Company does not allocate its assets by business segment as we evaluate underwriting results of each segment separately from the results of our investment portfolio. The Company uses underwriting ratios as measures of performance. The loss ratio is the ratio of losses and loss adjustment expenses to net earned premiums. The acquisition cost ratio is the ratio of acquisition costs to net earned premiums. The general and administrative expense ratio is the ratio of general and administrative expenses to net earned premiums. The combined ratio is the sum of the loss ratio, the acquisition cost ratio and the general and administrative expense ratio. The following tables provide a summary of gross and net written and earned premiums, underwriting income or loss, ratios and reserves for each of the Company’s business segments for the twelve months ended December 31, 2023, 2022 and 2021: Twelve Months Ended December 31, 2023 Reinsurance Insurance Total ($ in millions) Underwriting Revenues Gross written premiums $ 1,521.0 $ 2,446.6 $ 3,967.6 Net written premiums 1,098.0 1,483.9 2,581.9 Gross earned premiums 1,562.0 2,444.8 4,006.8 Net earned premiums 1,154.5 1,460.0 2,614.5 Underwriting Expenses Losses and loss adjustment expenses 611.1 941.9 1,553.0 Acquisition costs 208.6 171.6 380.2 General and administrative expenses 120.6 233.9 354.5 Underwriting income 214.2 112.6 326.8 Corporate and other expenses (1) (114.0) Non-operating expenses (2) (35.1) Net investment income 275.7 Realized and unrealized investment gains 75.9 Realized and unrealized investment losses (61.4) Change in fair value of derivatives 26.1 Interest expense (55.2) Net realized and unrealized foreign exchange (losses) (36.2) Income before income taxes 402.6 Income tax benefit 132.1 Net income $ 534.7 Net reserves for loss and loss adjustment expenses $ 1,373.1 $ 1,859.7 $ 3,232.8 Ratios Loss ratio 52.9 % 64.5 % 59.4 % Acquisition cost ratio 18.1 11.8 14.5 General and administrative expense ratio 10.4 16.0 13.6 Expense ratio 28.5 27.8 28.1 Combined ratio 81.4 % 92.3 % 87.5 % ________________ (1) Corporate and other expenses includes other income/(expenses), which were previously presented separately. (2) Non-operating expenses in the twelve months ended December 31, 2023 includes expenses in relation to consulting fees, non-recurring transformation program costs, and other non-recurring costs. Twelve Months Ended December 31, 2022 Reinsurance Insurance Total ($ in millions) Underwriting Revenues Gross written premiums $ 1,807.0 $ 2,531.7 $ 4,338.7 Net written premiums 1,426.4 1,469.6 2,896.0 Gross earned premiums 1,617.2 2,370.8 3,988.0 Net earned premiums 1,251.8 1,436.9 2,688.7 Underwriting Expenses Losses and loss adjustment expenses 770.3 909.7 1,680.0 Acquisition costs 252.4 179.4 431.8 General and administrative expenses 142.5 244.0 386.5 Underwriting income 86.6 103.8 190.4 Corporate expenses (71.7) Non-operating expenses (1) (36.0) Net investment income 188.1 Realized and unrealized investment gains 5.0 Realized and unrealized investment losses (182.6) Change in fair value of derivatives (80.5) Interest expense (43.7) Net realized and unrealized foreign exchange gains 15.9 Other income 8.2 Other expenses (20.1) (Loss) before income taxes (27.0) Income tax benefit 78.1 Net income $ 51.1 Net reserves for loss and loss adjustment expenses $ 1,360.7 $ 1,452.5 $ 2,813.2 Ratios Loss ratio 61.5 % 63.3 % 62.5 % Acquisition cost ratio 20.2 12.5 16.1 General and administrative expense ratio 11.4 17.0 14.4 Expense ratio 31.6 29.5 30.5 Combined ratio 93.1 % 92.8 % 93.0 % ________________ (1) Non-operating expenses in the twelve months ended December 31, 2022 includes expenses in relation to consulting fees, non-recurring transformation activities, and other non-recurring costs. Twelve Months Ended December 31, 2021 Reinsurance Insurance Total ($ in millions) Underwriting Revenues Gross written premiums $ 1,597.0 $ 2,341.4 $ 3,938.4 Net written premiums 1,199.0 1,388.7 2,587.7 Gross earned premiums 1,479.2 2,139.1 3,618.3 Net earned premiums 1,118.8 1,291.7 2,410.5 Underwriting Expenses Losses and loss adjustment expenses 705.2 988.1 1,693.3 Acquisition costs 221.6 192.5 414.1 General and administrative expenses 121.3 211.8 333.1 Underwriting income/(loss) 70.7 (100.7) (30.0) Corporate expenses (64.3) Non-operating expenses (1) (20.6) Net investment income 147.5 Realized and unrealized investment gains 56.2 Realized and unrealized investment losses (47.4) Change in fair value of derivatives (35.9) Interest expense (14.3) Net realized and unrealized foreign exchange gains 40.0 Other income 14.7 Other expenses (10.8) Income before income taxes 35.1 Income tax (expense) (5.3) Net income $ 29.8 Net reserves for loss and loss adjustment expenses $ 2,148.4 $ 2,165.3 $ 4,313.7 Ratios Loss ratio 63.0 % 76.5 % 70.2 % Acquisition cost ratio 19.8 14.9 17.2 General and administrative expense ratio 10.8 16.4 13.8 Expense ratio 30.6 31.3 31.0 Combined ratio 93.6 % 107.8 % 101.2 % ________________ (1) Non-operating expenses in the twelve months ended December 31, 2021 includes expenses in relation to consulting fees, non-recurring transformation activities, and other non-recurring costs. Geographical Areas . The following summary presents the Company’s gross written premiums based on the location of the insured risk for the twelve months ended December 31, 2023, 2022 and 2021. For the Twelve Months Ended December 31, 2023 December 31, 2022 December 31, 2021 ($ in millions) Australia/Asia $ 177.8 $ 257.5 $ 275.8 Europe 179.4 194.5 140.6 United Kingdom & Ireland 532.5 485.8 393.2 United States & Canada (1) 2,472.0 2,715.7 2,301.8 Worldwide excluding United States (2) 28.8 24.2 31.5 Worldwide including United States (3) 417.2 541.7 592.2 Other (4) 159.9 119.3 203.3 Total $ 3,967.6 $ 4,338.7 $ 3,938.4 ________________ (1) “United States and Canada” comprises individual policies that insure risks specifically in the United States and/or Canada, but not elsewhere. (2) “Worldwide excluding the United States” consists of individual policies that insure global risks with the specific exclusion of the United States. (3) “Worldwide including the United States” consists of individual policies that insure global risks with the specific inclusion of the United States. (4)

Investments

Investments	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Investments, Debt and Equity Securities [Abstract]
Investments	For the Twelve Months Ended December 31, 2023 December 31, 2022 December 31, 2021 ($ in millions) Fixed income securities — Available for sale $ 115.7 $ 95.6 $ 87.2 Fixed income securities — Trading 98.9 57.0 30.2 Short-term investments — Available for sale 5.3 0.6 0.1 Short-term investments — Trading 0.3 0.1 — Fixed term deposits (included in cash and cash equivalents) 39.9 6.6 0.7 Catastrophe bonds — Trading 0.2 0.3 0.9 Privately-held investments — Available for sale 0.1 — — Privately-held investments — Trading 44.7 24.3 18.2 Other investments, at fair value (1) (17.8) 13.9 21.9 Total 287.3 198.4 159.2 Investment expenses (11.6) (10.3) (11.7) Net investment income $ 275.7 $ 188.1 $ 147.5 ________________ (1) Other investments primarily represent the Company’s investments in investment funds. The amount reported represents the change in fair value of the investments in the period. The following table summarizes the net realized and unrealized investment gains and losses recorded in the consolidated statement of operations and the change in unrealized gains and losses on investments recorded in other comprehensive income for the twelve months ended December 31, 2023, 2022 and 2021: For the Twelve Months Ended December 31, 2023 December 31, 2022 December 31, 2021 ($ in millions) Available for sale: Fixed income securities — gross realized gains $ 1.6 $ 2.9 $ 22.7 Fixed income securities — gross realized (losses) (41.5) (58.4) (3.6) Short-term investments — gross realized gains 0.6 1.0 2.0 Short-term investments — gross realized (losses) (0.9) (0.5) (0.8) Net change in expected credit gains/(losses) 4.8 (5.0) (2.5) Trading: Fixed income securities — gross realized gains 1.0 0.2 12.2 Fixed income securities — gross realized (losses) (3.5) (1.8) (2.0) Short-term investments — gross realized gains 0.1 — 0.1 Short-term investments — gross realized (losses) (0.3) — (0.3) Privately-held investments — gross realized gains 0.8 0.7 0.6 Privately-held investments — gross realized (losses) — (0.1) (13.8) Privately-held investments — net unrealized (losses)/gains (15.2) (2.5) 18.1 Catastrophe bonds — net unrealized gains/(losses) 0.1 0.2 (0.8) Fixed income securities — net unrealized gains/(losses) 65.9 (113.9) (23.4) Investments — equity method: Gross realized and unrealized gains in MVI 0.2 — 0.1 Gross unrealized gains/(losses) in Multi-Line Reinsurer 0.8 (0.4) 0.2 Total net realized and unrealized investment gains/(losses) recorded in the consolidated statement of operations $ 14.5 $ (177.6) $ 8.8 Change in available for sale net unrealized gains/(losses): Available for sale investments $ 126.2 $ (391.7) $ (157.6) Income tax (expense)/benefit (20.6) 23.9 (0.3) Total change in net unrealized gains/(losses), net of taxes recorded in other comprehensive income $ 105.6 $ (367.8) $ (157.9) Balance Sheet Fixed Income Securities, Short-Term Investments and Privately-Held Investments — Available For Sale. The following tables present the cost or amortized cost, gross unrealized gains and losses and estimated fair market value of available for sale investments in fixed income securities, short-term investments and privately-held investments as at December 31, 2023 and December 31, 2022: As at December 31, 2023 Cost or Gross Gross Allowance for Credit Losses (1) Fair Market ($ in millions) Fixed Income Securities — Available for sale U.S. government $ 1,224.9 $ 4.4 $ (26.7) $ — $ 1,202.6 U.S. agency 7.5 — (0.3) — 7.2 Municipal 133.6 — (5.0) (0.5) 128.1 Corporate 2,051.1 12.1 (101.5) (2.4) 1,959.3 Non-U.S. government-backed corporate 106.5 0.1 (5.9) — 100.7 Non-U.S. government 279.9 0.6 (6.7) — 273.8 Agency commercial mortgage-backed 6.6 — (0.8) — 5.8 Agency residential mortgage-backed 519.9 0.1 (74.9) — 445.1 Total fixed income securities — Available for sale 4,330.0 17.3 (221.8) (2.9) 4,122.6 Short-term investments — Available for sale 93.6 — — — 93.6 Privately-held investments — Available for sale Asset-backed securities 14.7 0.2 — — 14.9 Total Investments — Available for sale $ 4,438.3 $ 17.5 $ (221.8) $ (2.9) $ 4,231.1 ________________ (1) For more information on the allowance for expected credit losses, refer to Note 25, “Allowance for Expected Credit Losses”. As at December 31, 2022 Cost or Gross Gross Allowance for Credit Losses Fair Market ($ in millions) Fixed Income Securities — Available for sale U.S. government $ 1,003.7 $ — $ (50.7) $ — $ 953.0 U.S. agency 9.2 — (0.4) — 8.8 Municipal 159.9 — (9.4) (1.0) 149.5 Corporate 2,016.9 3.5 (169.1) (6.3) 1,845.0 Non-U.S. government-backed corporate 119.4 — (8.8) (0.2) 110.4 Non-U.S. government 225.2 0.1 (11.5) (0.2) 213.6 Non-agency commercial mortgage-backed 6.6 — (1.0) — 5.6 Agency mortgage-backed 590.4 — (87.7) — 502.7 Total fixed income securities — Available for sale 4,131.3 3.6 (338.6) (7.7) 3,788.6 Short-term investments — Available for sale 52.4 — (0.4) — 52.0 Total Investments — Available for sale $ 4,183.7 $ 3.6 $ (339.0) $ (7.7) $ 3,840.6 Fixed Income Securities, Short-Term Investments, Equities, Catastrophe Bonds and Privately-Held Investments — Trading . The following tables present the cost or amortized cost, gross unrealized gains and losses, and estimated fair market value of trading investments in fixed income securities, short-term investments, catastrophe bonds and privately-held investments as at December 31, 2023 and December 31, 2022: As at December 31, 2023 Cost or Gross Gross Fair Market ($ in millions) Fixed Income Securities — Trading U.S. government $ 248.7 $ 0.5 $ (3.7) $ 245.5 Municipal 3.3 — (0.2) 3.1 Corporate 178.8 0.7 (8.0) 171.5 High yield loans 90.8 1.3 — 92.1 Non-U.S. government-backed corporate 8.6 — (0.3) 8.3 Non-U.S. government 35.8 0.1 (1.1) 34.8 Asset-backed 936.0 2.1 (29.9) 908.2 Agency mortgage-backed 25.0 — (2.8) 22.2 Total fixed income securities — Trading 1,527.0 4.7 (46.0) 1,485.7 Short-term investments — Trading 2.1 — — 2.1 Catastrophe bonds — Trading 1.6 — — 1.6 Privately-held investments — Tradin g Commercial mortgage loans 293.2 1.0 (19.3) 274.9 Middle market loans and other private debt 85.9 — (1.1) 84.8 Asset-backed securities 83.1 0.4 (0.6) 82.9 Global corporate securities 14.7 — (0.3) 14.4 Short-term investments 18.0 — — 18.0 Total privately-held investments — Trading 494.9 1.4 (21.3) 475.0 Total Investments — Trading $ 2,025.6 $ 6.1 $ (67.3) $ 1,964.4 As at December 31, 2022 Cost or Gross Gross Fair Market ($ in millions) Fixed Income Securities — Trading U.S. government $ 267.9 $ — $ (6.3) $ 261.6 Municipal 3.9 — (0.3) 3.6 Corporate 175.3 0.3 (13.5) 162.1 High yield loans 90.2 0.2 (2.1) 88.3 Non-U.S. government-backed corporate 12.2 — (0.6) 11.6 Non-U.S. government 32.2 — (1.8) 30.4 Asset-backed 970.3 0.2 (74.0) 896.5 Agency mortgage-backed 24.7 — (3.3) 21.4 Total fixed income securities — Trading 1,576.7 0.7 (101.9) 1,475.5 Short-term investments — Trading 6.3 — — 6.3 Catastrophe bonds — Trading 5.1 — (2.2) 2.9 Privately-held investments — Trading Commercial mortgage loans 312.6 0.6 (1.1) 312.1 Middle market loans and other private debt 109.0 — (2.1) 106.9 Asset-backed securities 68.8 — (2.0) 66.8 Global corporate securities 15.1 — (0.1) 15.0 Equity securities 6.6 — — 6.6 Short-term investments 25.6 — — 25.6 Total privately-held investments — Trading 537.7 0.6 (5.3) 533.0 Total Investments — Trading $ 2,125.8 $ 1.3 $ (109.4) $ 2,017.7 Catastrophe bonds. The Company has invested in catastrophe bonds with a total value of $1.6 million as at December 31, 2023 (December 31, 2022 — $2.9 million). The bonds are either zero-coupon notes or receive quarterly interest payments based on variable interest rates with scheduled maturities ranging from 2024 to 2026. The redemption value of the bonds will adjust based on the occurrence or aggregate occurrence of a covered event, such as windstorms and earthquakes in the United States, Canada, the North Atlantic, South America, Europe, Japan or Australia. Privately-held investments. The Company has invested in privately-held investments, which primarily include commercial mortgage loans of $274.9 million and middle market loans and other private debt of $84.8 million as at December 31, 2023 (December 31, 2022 — commercial mortgage loans of $312.1 million; middle market loans and other private debt of $106.9 million). Privately-held investments also includes investments in asset-backed securities, global corporate securities, and other short term investments. Commercial Mortgage Loans . The commercial mortgage loans are related to investments in properties including apartments, hotels, office and retail buildings, other commercial properties and industrial properties. The commercial mortgage loan portfolio is diversified by property type, geographic region and issuer to reduce risks. As part of our investment process, we evaluate factors such as size, property type, and security to determine that properties are performing at a consistent and acceptable level to secure the related debt. Middle Market Loans and Other Private Debt . The middle market loans are investments in senior secured loan positions with full covenants, focused on the middle market in both U.S., Europe and the Caribbean. The other private debt consists of debt securities issued to private investment funds. The middle market loan and other private debt portfolio is diversified by industry type, geographic region and issuer to reduce risks. As part of our investment process, we evaluate factors such as size, industry and security to determine that loans are performing at a consistent and acceptable level to secure the related debt. Asset-backed securities . Asset-backed securities represent interests in underlying pools of diversified referenced assets that are collateralized and backed by future cash flows and these securities are performing. Global corporate securities. The global corporates portfolio consists of debt securities with a non-U.S. debt issuer. Investments — Equity Method. In January 2015, the Company, along with seven other insurance companies, established a micro-insurance venture consortium and micro-insurance incubator (“MVI”) domiciled in Bermuda. The MVI is a social impact organization that provides micro-insurance products to assist global emerging consumers. In March 2021, the Company committed an additional $0.8 million equity contribution to MVI over a 2 year period and paid $0.4 million in the period ending December 31, 2022. On January 1, 2017, the Company purchased through its wholly-owned subsidiary, Aspen U.S. Holdings, Inc. (“Aspen U.S. Holdings”), a 49% share of Digital Risk Resources, LLC (“Digital Re”), a U.S.-based enterprise engaged in the business of developing, marketing and servicing turnkey information security and privacy liability insurance products for a total consideration of $2.3 million. The investment is accounted for under the equity method and adjustments to the carrying value of this investment are made based on the Company’s share of capital, including share of income and expenses. On December 23, 2019, the Company committed $5.0 million as an equity investment in the holding company of a multi-line reinsurer. The strategy for the multi-line reinsurer is to combine a diversified reinsurance business, focused primarily on long-tailed lines of property and casualty business and, potentially to a lesser extent, life business, with a diversified investment strategy. During the period ending December 31, 2023, $0.4 million (December 31, 2022 — $1.6 million) of capital was invested in the multi-line reinsurer. The table below shows the Company’s investments in MVI, Multi-Line Reinsurer and Digital Re for the twelve months ended December 31, 2023 and 2022:	Investments Income Statement Net investment income . The following table summarizes investment income for the twelve months ended December 31, 2023, 2022 and 2021: For the Twelve Months Ended December 31, 2023 December 31, 2022 December 31, 2021 ($ in millions) Fixed income securities — Available for sale $ 115.7 $ 95.6 $ 87.2 Fixed income securities — Trading 98.9 57.0 30.2 Short-term investments — Available for sale 5.3 0.6 0.1 Short-term investments — Trading 0.3 0.1 — Fixed term deposits (included in cash and cash equivalents) 39.9 6.6 0.7 Catastrophe bonds — Trading 0.2 0.3 0.9 Privately-held investments — Available for sale 0.1 — — Privately-held investments — Trading 44.7 24.3 18.2 Other investments, at fair value (1) (17.8) 13.9 21.9 Total 287.3 198.4 159.2 Investment expenses (11.6) (10.3) (11.7) Net investment income $ 275.7 $ 188.1 $ 147.5 ________________ (1) Other investments primarily represent the Company’s investments in investment funds. The amount reported represents the change in fair value of the investments in the period. The following table summarizes the net realized and unrealized investment gains and losses recorded in the consolidated statement of operations and the change in unrealized gains and losses on investments recorded in other comprehensive income for the twelve months ended December 31, 2023, 2022 and 2021: For the Twelve Months Ended December 31, 2023 December 31, 2022 December 31, 2021 ($ in millions) Available for sale: Fixed income securities — gross realized gains $ 1.6 $ 2.9 $ 22.7 Fixed income securities — gross realized (losses) (41.5) (58.4) (3.6) Short-term investments — gross realized gains 0.6 1.0 2.0 Short-term investments — gross realized (losses) (0.9) (0.5) (0.8) Net change in expected credit gains/(losses) 4.8 (5.0) (2.5) Trading: Fixed income securities — gross realized gains 1.0 0.2 12.2 Fixed income securities — gross realized (losses) (3.5) (1.8) (2.0) Short-term investments — gross realized gains 0.1 — 0.1 Short-term investments — gross realized (losses) (0.3) — (0.3) Privately-held investments — gross realized gains 0.8 0.7 0.6 Privately-held investments — gross realized (losses) — (0.1) (13.8) Privately-held investments — net unrealized (losses)/gains (15.2) (2.5) 18.1 Catastrophe bonds — net unrealized gains/(losses) 0.1 0.2 (0.8) Fixed income securities — net unrealized gains/(losses) 65.9 (113.9) (23.4) Investments — equity method: Gross realized and unrealized gains in MVI 0.2 — 0.1 Gross unrealized gains/(losses) in Multi-Line Reinsurer 0.8 (0.4) 0.2 Total net realized and unrealized investment gains/(losses) recorded in the consolidated statement of operations $ 14.5 $ (177.6) $ 8.8 Change in available for sale net unrealized gains/(losses): Available for sale investments $ 126.2 $ (391.7) $ (157.6) Income tax (expense)/benefit (20.6) 23.9 (0.3) Total change in net unrealized gains/(losses), net of taxes recorded in other comprehensive income $ 105.6 $ (367.8) $ (157.9) Balance Sheet Fixed Income Securities, Short-Term Investments and Privately-Held Investments — Available For Sale. The following tables present the cost or amortized cost, gross unrealized gains and losses and estimated fair market value of available for sale investments in fixed income securities, short-term investments and privately-held investments as at December 31, 2023 and December 31, 2022: As at December 31, 2023 Cost or Gross Gross Allowance for Credit Losses (1) Fair Market ($ in millions) Fixed Income Securities — Available for sale U.S. government $ 1,224.9 $ 4.4 $ (26.7) $ — $ 1,202.6 U.S. agency 7.5 — (0.3) — 7.2 Municipal 133.6 — (5.0) (0.5) 128.1 Corporate 2,051.1 12.1 (101.5) (2.4) 1,959.3 Non-U.S. government-backed corporate 106.5 0.1 (5.9) — 100.7 Non-U.S. government 279.9 0.6 (6.7) — 273.8 Agency commercial mortgage-backed 6.6 — (0.8) — 5.8 Agency residential mortgage-backed 519.9 0.1 (74.9) — 445.1 Total fixed income securities — Available for sale 4,330.0 17.3 (221.8) (2.9) 4,122.6 Short-term investments — Available for sale 93.6 — — — 93.6 Privately-held investments — Available for sale Asset-backed securities 14.7 0.2 — — 14.9 Total Investments — Available for sale $ 4,438.3 $ 17.5 $ (221.8) $ (2.9) $ 4,231.1 ________________ (1) For more information on the allowance for expected credit losses, refer to Note 25, “Allowance for Expected Credit Losses”. As at December 31, 2022 Cost or Gross Gross Allowance for Credit Losses Fair Market ($ in millions) Fixed Income Securities — Available for sale U.S. government $ 1,003.7 $ — $ (50.7) $ — $ 953.0 U.S. agency 9.2 — (0.4) — 8.8 Municipal 159.9 — (9.4) (1.0) 149.5 Corporate 2,016.9 3.5 (169.1) (6.3) 1,845.0 Non-U.S. government-backed corporate 119.4 — (8.8) (0.2) 110.4 Non-U.S. government 225.2 0.1 (11.5) (0.2) 213.6 Non-agency commercial mortgage-backed 6.6 — (1.0) — 5.6 Agency mortgage-backed 590.4 — (87.7) — 502.7 Total fixed income securities — Available for sale 4,131.3 3.6 (338.6) (7.7) 3,788.6 Short-term investments — Available for sale 52.4 — (0.4) — 52.0 Total Investments — Available for sale $ 4,183.7 $ 3.6 $ (339.0) $ (7.7) $ 3,840.6 Fixed Income Securities, Short-Term Investments, Equities, Catastrophe Bonds and Privately-Held Investments — Trading . The following tables present the cost or amortized cost, gross unrealized gains and losses, and estimated fair market value of trading investments in fixed income securities, short-term investments, catastrophe bonds and privately-held investments as at December 31, 2023 and December 31, 2022: As at December 31, 2023 Cost or Gross Gross Fair Market ($ in millions) Fixed Income Securities — Trading U.S. government $ 248.7 $ 0.5 $ (3.7) $ 245.5 Municipal 3.3 — (0.2) 3.1 Corporate 178.8 0.7 (8.0) 171.5 High yield loans 90.8 1.3 — 92.1 Non-U.S. government-backed corporate 8.6 — (0.3) 8.3 Non-U.S. government 35.8 0.1 (1.1) 34.8 Asset-backed 936.0 2.1 (29.9) 908.2 Agency mortgage-backed 25.0 — (2.8) 22.2 Total fixed income securities — Trading 1,527.0 4.7 (46.0) 1,485.7 Short-term investments — Trading 2.1 — — 2.1 Catastrophe bonds — Trading 1.6 — — 1.6 Privately-held investments — Tradin g Commercial mortgage loans 293.2 1.0 (19.3) 274.9 Middle market loans and other private debt 85.9 — (1.1) 84.8 Asset-backed securities 83.1 0.4 (0.6) 82.9 Global corporate securities 14.7 — (0.3) 14.4 Short-term investments 18.0 — — 18.0 Total privately-held investments — Trading 494.9 1.4 (21.3) 475.0 Total Investments — Trading $ 2,025.6 $ 6.1 $ (67.3) $ 1,964.4 As at December 31, 2022 Cost or Gross Gross Fair Market ($ in millions) Fixed Income Securities — Trading U.S. government $ 267.9 $ — $ (6.3) $ 261.6 Municipal 3.9 — (0.3) 3.6 Corporate 175.3 0.3 (13.5) 162.1 High yield loans 90.2 0.2 (2.1) 88.3 Non-U.S. government-backed corporate 12.2 — (0.6) 11.6 Non-U.S. government 32.2 — (1.8) 30.4 Asset-backed 970.3 0.2 (74.0) 896.5 Agency mortgage-backed 24.7 — (3.3) 21.4 Total fixed income securities — Trading 1,576.7 0.7 (101.9) 1,475.5 Short-term investments — Trading 6.3 — — 6.3 Catastrophe bonds — Trading 5.1 — (2.2) 2.9 Privately-held investments — Trading Commercial mortgage loans 312.6 0.6 (1.1) 312.1 Middle market loans and other private debt 109.0 — (2.1) 106.9 Asset-backed securities 68.8 — (2.0) 66.8 Global corporate securities 15.1 — (0.1) 15.0 Equity securities 6.6 — — 6.6 Short-term investments 25.6 — — 25.6 Total privately-held investments — Trading 537.7 0.6 (5.3) 533.0 Total Investments — Trading $ 2,125.8 $ 1.3 $ (109.4) $ 2,017.7 Catastrophe bonds. The Company has invested in catastrophe bonds with a total value of $1.6 million as at December 31, 2023 (December 31, 2022 — $2.9 million). The bonds are either zero-coupon notes or receive quarterly interest payments based on variable interest rates with scheduled maturities ranging from 2024 to 2026. The redemption value of the bonds will adjust based on the occurrence or aggregate occurrence of a covered event, such as windstorms and earthquakes in the United States, Canada, the North Atlantic, South America, Europe, Japan or Australia. Privately-held investments. The Company has invested in privately-held investments, which primarily include commercial mortgage loans of $274.9 million and middle market loans and other private debt of $84.8 million as at December 31, 2023 (December 31, 2022 — commercial mortgage loans of $312.1 million; middle market loans and other private debt of $106.9 million). Privately-held investments also includes investments in asset-backed securities, global corporate securities, and other short term investments. Commercial Mortgage Loans . The commercial mortgage loans are related to investments in properties including apartments, hotels, office and retail buildings, other commercial properties and industrial properties. The commercial mortgage loan portfolio is diversified by property type, geographic region and issuer to reduce risks. As part of our investment process, we evaluate factors such as size, property type, and security to determine that properties are performing at a consistent and acceptable level to secure the related debt. Middle Market Loans and Other Private Debt . The middle market loans are investments in senior secured loan positions with full covenants, focused on the middle market in both U.S., Europe and the Caribbean. The other private debt consists of debt securities issued to private investment funds. The middle market loan and other private debt portfolio is diversified by industry type, geographic region and issuer to reduce risks. As part of our investment process, we evaluate factors such as size, industry and security to determine that loans are performing at a consistent and acceptable level to secure the related debt. Asset-backed securities . Asset-backed securities represent interests in underlying pools of diversified referenced assets that are collateralized and backed by future cash flows and these securities are performing. Global corporate securities. The global corporates portfolio consists of debt securities with a non-U.S. debt issuer. Investments — Equity Method. In January 2015, the Company, along with seven other insurance companies, established a micro-insurance venture consortium and micro-insurance incubator (“MVI”) domiciled in Bermuda. The MVI is a social impact organization that provides micro-insurance products to assist global emerging consumers. In March 2021, the Company committed an additional $0.8 million equity contribution to MVI over a 2 year period and paid $0.4 million in the period ending December 31, 2022. On January 1, 2017, the Company purchased through its wholly-owned subsidiary, Aspen U.S. Holdings, Inc. (“Aspen U.S. Holdings”), a 49% share of Digital Risk Resources, LLC (“Digital Re”), a U.S.-based enterprise engaged in the business of developing, marketing and servicing turnkey information security and privacy liability insurance products for a total consideration of $2.3 million. The investment is accounted for under the equity method and adjustments to the carrying value of this investment are made based on the Company’s share of capital, including share of income and expenses. On December 23, 2019, the Company committed $5.0 million as an equity investment in the holding company of a multi-line reinsurer. The strategy for the multi-line reinsurer is to combine a diversified reinsurance business, focused primarily on long-tailed lines of property and casualty business and, potentially to a lesser extent, life business, with a diversified investment strategy. During the period ending December 31, 2023, $0.4 million (December 31, 2022 — $1.6 million) of capital was invested in the multi-line reinsurer. The table below shows the Company’s investments in MVI, Multi-Line Reinsurer and Digital Re for the twelve months ended December 31, 2023 and 2022: MVI Multi-Line Reinsurer Digital Re Total ($ in millions) Opening undistributed value of investment as at January 1, 2023 $ 0.8 $ 5.2 $ 0.2 $ 6.2 Investment in the period — 0.4 — 0.4 Distribution received — — — — Unrealized gain for the twelve months to December 31, 2023 0.2 0.8 — 1.0 Closing value of investment as at December 31, 2023 1.0 6.4 0.2 7.6 Opening undistributed value of investment as at January 1, 2022 $ 0.5 $ 3.2 $ 0.2 $ 3.9 Investment in the period 0.4 1.6 — 2.0 Distribution received — — — — Unrealized (loss)/gain for the twelve months to December 31, 2022 (0.1) 0.4 — 0.3 Closing value of investments at December 31, 2022 $ 0.8 $ 5.2 $ 0.2 $ 6.2 Other Investments. On December 20, 2017, the Company committed to, and during 2018 invested $100.0 million as a limited partner to a real estate fund, classified as other investments. As at December 31, 2023, the current fair value of the fund is $117.1 million (2022 — $134.6 million). During 2020, the Company committed $10.5 million as a limited partner to a related party managed lending fund. The partnership was established to provide direct lending to large corporate borrowers. On April 1, 2021, the Company committed an additional $2.8 million to the fund. As at December 31, 2023, the current fair value of the fund is $15.9 million (2022 — $12.7 million) and the unfunded commitment is $1.1 million. On April 1, 2021, the Company established pledge accounts with its custodian bank for the ability to obtain liquidity and funding services provided by a U.S. co-operative bank, which provides liquidity and funding to its insurance member institutions. As at December 31, 2023, the fair value of the Company’s member shares in the bank is $1.7 million (2022 — $1.3 million). On September 30, 2021, the Company committed $20.0 million as a limited partner to a third-party managed real estate fund. The Partnership was established to make equity and equity related investments in multifamily and other commercial real estate properties located in the United States and its territories, with the goal of generating superior risk-adjusted returns. The Partnership seeks to acquire commercial real estate assets including real estate assets (or interests therein) that may have management or operational problems and require improvements or lack sufficient capital, including mortgage loans and development or redevelopment properties. On April 1, 2022, the Company committed an additional $10.0 million to the fund. As at December 31, 2023, the current fair value of the fund is $39.8 million (2022 — $40.6 million) and the unfunded commitment is $2.2 million. On April 1, 2022, the Company committed $30.0 million as a limited partner to a related party managed real estate fund. The Partnership was established to pursue investment opportunities to acquire, recapitalize, restructure and reposition real estate assets, portfolios and companies primarily in the United States. As at December 31, 2023, the current fair value of the fund is $23.9 million (2022 — $25.3 million)and the unfunded commitment is $4.1 million. On May 5, 2022, the Company committed $15.0 million as a limited partner to a third-party managed infrastructure fund. The Partnership was established to make investments in value added infrastructure investments in environmental services, transportation, communications and digital, energy/energy transition and other infrastructure sectors primarily in North America. As at December 31, 2023, the current fair value of the fund is $10.8 million (2022 — $8.2 million) and the unfunded commitment is $4.0 million. On August 31, 2023, the Company committed £7.0 million as a limited partner to a third-party managed debt fund. The fund will focus on three core sectors - health and social care, affordable housing, and social infrastructure. The fund will invest across the U.K., focusing on areas of poverty and deprivation. The fund provides fixed-rate loans typically backed by property assets. Borrowers will be established, socially impactful organizations, with a history of profitable revenue generation. As at December 31, 2023, the current fair value of the fund is $0.1 million (2022 — $Nil) and the unfunded commitment is £6.9 million. As at December 31, 2023, the aggregate current fair value of the investment funds described above is $209.3 million (2022 — $221.3 million). For further information on the investment funds, refer to Note 21(a) in these consolidated financial statements, “Commitments and Contingent Liabilities.” Fixed Income Securities, Short-Term Investments and Privately-Held Investments — Available for sale. The scheduled maturity distribution of the Company’s available for sale securities as at December 31, 2023 and December 31, 2022 is set forth below. Actual maturities may differ from contractual maturities because issuers of securities may have the right to call or prepay obligations with or without call or prepayment penalties. As at December 31, 2023 Amortized Fair Market ($ in millions) Due one year or less $ 512.0 $ 505.8 Due after one year through five years 2,889.0 2,818.9 Due after five years through ten years 495.1 440.3 Due after ten years 15.7 15.2 3,911.8 3,780.2 Agency commercial mortgage-backed 6.6 5.8 Agency residential mortgage-backed 519.9 445.1 Total Investments — Available for sale $ 4,438.3 $ 4,231.1 At December 31, 2022 Amortized Fair Market ($ in millions) Due one year or less $ 231.5 $ 228.0 Due after one year through five years 2,526.9 2,385.7 Due after five years through ten years 815.1 707.7 Due after ten years 13.2 10.9 3,586.7 3,332.3 Non-agency commercial mortgage-backed 6.6 5.6 Agency mortgage-backed 590.4 502.7 Total Investments — Available for sale $ 4,183.7 $ 3,840.6 Guaranteed Investments. As at December 31, 2023 and December 31, 2022, the Company held no investments which are guaranteed by mono-line insurers, excluding those with explicit government guarantees. The Company’s exposure to other third-party guaranteed debt is primarily to investments backed by non-U.S. government guaranteed issuers. Gross unrealized losses, available for sale. The following tables summarize, by type of security, the aggregate fair value and gross unrealized loss by length of time the security has been in an unrealized loss position for the Company’s available for sale portfolio as at December 31, 2023 and December 31, 2022: December 31, 2023 0-12 months Over 12 months Total Fair Gross Fair Gross Fair Gross Number of ($ in millions) U.S. government $ 105.5 $ (0.5) $ 673.3 $ (26.2) $ 778.8 $ (26.7) 74 U.S. agency — — 7.2 (0.3) 7.2 (0.3) 1 Municipal 11.1 (1.0) 117.0 (4.0) 128.1 (5.0) 54 Corporate 46.7 (0.4) 1,287.2 (101.1) 1,333.9 (101.5) 558 Non-U.S. government-backed corporate 0.2 — 95.5 (5.9) 95.7 (5.9) 12 Non-U.S. government 32.9 (0.2) 180.1 (6.5) 213.0 (6.7) 53 Agency commercial mortgage-backed — — 5.8 (0.8) 5.8 (0.8) 1 Agency residential mortgage-backed 0.1 — 435.9 (74.9) 436.0 (74.9) 197 Total $ 196.5 $ (2.1) $ 2,802.0 $ (219.7) $ 2,998.5 $ (221.8) 950 December 31, 2022 0-12 months Over 12 months Total Fair Gross Fair Gross Fair Gross Number of ($ in millions) U.S. government $ 741.1 $ (30.6) $ 203.4 $ (20.1) $ 944.5 $ (50.7) 118 U.S. agency 7.1 (0.4) 1.7 — 8.8 (0.4) 5 Municipal 133.1 (7.1) 16.4 (2.3) 149.5 (9.4) 59 Corporate 1,104.7 (77.6) 568.2 (91.5) 1,672.9 (169.1) 701 Non-U.S. government-backed corporate 11.3 (0.5) 99.0 (8.3) 110.3 (8.8) 16 Non-U.S. government 63.8 (2.9) 135.9 (8.6) 199.7 (11.5) 45 Non-agency commercial mortgage-backed securities 5.6 (1.0) — — 5.6 (1.0) 1 Agency mortgage-backed 202.8 (24.9) 299.1 (62.8) 501.9 (87.7) 220 Total fixed income securities — Available for sale 2,269.5 (145.0) 1,323.7 (193.6) 3,593.2 (338.6) 1,165 Total short-term investments — Available for sale 51.9 (0.4) — — 51.9 (0.4) 1 Total $ 2,321.4 $ (145.4) $ 1,323.7 $ (193.6) $ 3,645.1 $ (339.0) 1,166 At December 31, 2023, 950 available for sale securities were in an unrealized loss position of $221.8 million, of which $1.2 million was related to securities below investment grade or not rated. The unrealized losses of $221.8 million were due to non-credit factors and are expected to be recovered as the related securities approach maturity. The Company does not intend to sell the securities in an unrealized loss position and it is not more likely than not that the Company will be required to sell the investments before recovery of their amortized cost bases.

Variable Interest Entities

Variable Interest Entities	12 Months Ended
	Dec. 31, 2023
Payables and Accruals [Abstract]
Variable Interest Entities	Variable Interest Entities As at December 31, 2023, the Company held an investment in one (December 31, 2022 — one) variable interest entity (“VIE”), namely Peregrine Reinsurance Ltd (“Peregrine”). Peregrine. In November 2016, Peregrine, a subsidiary of the Company, was registered as a segregated accounts company under the Segregated Accounts Companies Act 2000, as amended. As at December 31, 2023, Peregrine had six segregated accounts which were funded by third-party investors, which are not consolidated, and two segregated accounts which are funded by Aspen and are consolidated within the financial statements. The Company has determined that Peregrine has the characteristics of a VIE as addressed by the guidance in ASC 810, Consolidation

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2023
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements The Company’s estimates of fair value for financial assets and liabilities are based on the framework established in the fair value accounting guidance included in ASC Topic 820, “ Fair Value Measurements and Disclosures .” The framework prioritizes the inputs, which refer broadly to assumptions market participants would use in pricing an asset or liability, into three levels. The Company considers prices for actively traded securities to be derived based on quoted prices in an active market for identical assets, which are Level 1 inputs in the fair value hierarchy. The majority of these securities are valued using prices supplied by pricing services. The Company considers prices for other securities that may not be as actively traded which are priced via pricing services, vendors and broker-dealers, or with reference to interest rates and yield curves, to be derived based on inputs that are observable for the asset, either directly or indirectly, which are Level 2 inputs in the fair value hierarchy. The majority of these securities are also valued using prices supplied by pricing services. The Company considers securities, other financial instruments, privately-held investments and derivative insurance contracts subject to fair value measurement whose valuation is derived by internal valuation models to be based largely on unobservable inputs, which are Level 3 inputs in the fair value hierarchy. The following tables present the level within the fair value hierarchy at which the Company’s financial assets and liabilities are measured on a recurring basis as at December 31, 2023 and December 31, 2022: As at December 31, 2023 Level 1 Level 2 Level 3 Total ($ in millions) Available for sale financial assets, at fair value U.S. government $ 1,202.6 $ — $ — $ 1,202.6 U.S. agency — 7.2 — 7.2 Municipal — 128.1 — 128.1 Corporate — 1,959.3 — 1,959.3 Non-U.S. government-backed corporate — 100.7 — 100.7 Non-U.S. government 200.4 73.4 — 273.8 Agency commercial mortgage-backed — 5.8 — 5.8 Agency residential mortgage-backed — 445.1 — 445.1 Total fixed income securities available for sale, at fair value 1,403.0 2,719.6 — 4,122.6 Short-term investments available for sale, at fair value 86.7 6.9 — 93.6 Privately-held investments available for sale, at fair value — — 14.9 14.9 Held for trading financial assets, at fair value U.S. government 245.5 — — 245.5 Municipal — 3.1 — 3.1 Corporate — 171.5 — 171.5 Non-U.S. government-backed corporate — 8.3 — 8.3 High yield loans — 92.1 — 92.1 Non-U.S. government 13.0 21.8 — 34.8 Asset-backed — 908.2 — 908.2 Agency mortgage-backed — 22.2 — 22.2 Total fixed income securities trading, at fair value 258.5 1,227.2 — 1,485.7 Short-term investments trading, at fair value 0.2 1.9 — 2.1 Catastrophe bonds trading, at fair value — 1.6 — 1.6 Privately-held investments trading, at fair value — — 475.0 475.0 Other investments (1) — — — 209.3 Other financial assets and liabilities, at fair value Derivative assets — foreign exchange contracts — 31.7 — 31.7 Derivative liabilities — foreign exchange contracts — (9.3) — (9.3) Derivative liabilities — loss portfolio transfer (2) — (16.5) (16.5) Total 1,748.4 3,979.6 473.4 6,410.7 ________________ (1) Other investments represents our investments in investment funds operating strategies in real estate, infrastructure and lending and are measured at fair value using the net asset value per share practical expedient. As a result the investments are not classified in the fair value hierarchy. The fair value amounts presented in the table above are intended to permit reconciliation of the fair value hierarchy to the amounts presented in the consolidated balance sheets. The investment in the funds is subject to restrictions as detailed in Note 21(a), “Commitments and Contingent Liabilities.” (2) The loss portfolio transfer contract includes a funds withheld arrangement that provides variable interest expense based on Aspen’s investment performance. As a result, the funds withheld arrangement is considered an embedded derivative and accounted for as an option-based derivative. At December 31, 2022 Level 1 Level 2 Level 3 Total ($ in millions) Available for sale financial assets, at fair value U.S. government $ 953.0 $ — $ — $ 953.0 U.S. agency — 8.8 — 8.8 Municipal — 149.5 — 149.5 Corporate — 1,845.0 — 1,845.0 Non-U.S. government-backed corporate — 110.4 — 110.4 Non-U.S. government 145.3 68.3 — 213.6 Non-agency commercial mortgage-backed — 5.6 — 5.6 Agency mortgage-backed — 502.7 — 502.7 Total fixed income securities available for sale, at fair value 1,098.3 2,690.3 — 3,788.6 Short-term investments available for sale, at fair value 51.9 0.1 — 52.0 Held for trading financial assets, at fair value U.S. government 261.6 — — 261.6 Municipal — 3.6 — 3.6 Corporate — 162.1 — 162.1 Non-U.S. government-backed corporate — 11.6 — 11.6 High yield loans — 88.3 — 88.3 Non-U.S. government 10.6 19.8 — 30.4 Asset-backed — 896.5 — 896.5 Agency mortgage-backed — 21.4 — 21.4 Total fixed income securities trading, at fair value 272.2 1,203.3 — 1,475.5 Short-term investments trading, at fair value 6.3 — — 6.3 Catastrophe bonds trading, at fair value — 2.9 — 2.9 Privately-held investments — — 533.0 533.0 Other investments (1) — — — 221.3 Other financial assets and liabilities, at fair value Derivative assets — foreign exchange contracts — 56.2 — 56.2 Derivative liabilities — foreign exchange contracts — (3.2) — (3.2) Derivative liabilities — loss portfolio transfer (2) — — (31.7) (31.7) Total $ 1,428.7 $ 3,949.6 $ 501.3 $ 6,100.9 ________________ (1) Other investments represents our investments in investment funds operating strategies in real estate, infrastructure and lending and are measured at fair value using the net asset value per share practical expedient. As a result the investments are not classified in the fair value hierarchy. The fair value amounts presented in the table above are intended to permit reconciliation of the fair value hierarchy to the amounts presented in the consolidated balance sheets. The investment in the funds is subject to restrictions as detailed in Note 21(a), “Commitments and Contingent Liabilities.” (2) The loss portfolio transfer contract includes a funds withheld arrangement that provides variable interest expense based on Aspen’s investment performance. As a result, the funds withheld arrangement is considered an embedded derivative and accounted for as an option-based derivative. Transfers of assets into or out of a particular level are recorded at their fair values as of the end of each reporting period consistent with the date of the determination of fair value. During the twelve months ended December 31, 2023, $12.1 million was transferred out of Level 3 and $5.3 million transferred into Level 3 (December 31, 2022 —$Nil was transferred in or out of Level 3). The transfers out of Level 3, and into Level 2, was due to the availability of observable market inputs. The following table presents a reconciliation of the beginning and ending balances for all assets and liabilities measured at fair value on a recurring basis using Lev el 3 inputs for the twelve months ended December 31, 2023 and December 31, 2022: Twelve Months Ended December 31, 2023 Balance at beginning of year Purchases and issuances Transfers in Transfers (out) Settlements and sales Increase/(decrease) in fair value included in net income (1) / OCI (2) Balance at end of year Change in unrealized gains (losses) relating to assets held at end of year (1) (2) Assets Privately-held investments — available for sale Asset-backed securities $ — $ 14.7 $ — $ — $ — $ 0.2 $ 14.9 $ 0.2 Privately-held investments — trading Commercial mortgage loans $ 312.1 $ 40.6 $ — $ — $ (61.5) $ (16.3) $ 274.9 $ (17.9) Middle market loans and other private debt 106.9 18.3 — — (41.9) 1.5 84.8 0.5 Asset-backed securities 66.8 21.9 5.3 (5.5) (7.6) 2.0 82.9 1.7 Global corporate securities 15.0 — — — (0.4) (0.2) 14.4 (0.2) Equity securities 6.6 — — (6.6) — — — — Short-term investments 25.6 18.2 — — (25.8) — 18.0 — Total Level 3 assets $ 533.0 $ 113.7 $ 5.3 $ (12.1) $ (137.2) $ (12.8) $ 489.9 $ (15.7) Liabilities Derivative liabilities — loss portfolio transfer $ (31.7) $ — $ — $ — $ — $ 15.2 $ (16.5) $ 15.2 Total Level 3 liabilities $ (31.7) $ — $ — $ — $ — $ 15.2 $ (16.5) $ 15.2 Twelve Months Ended December 31, 2022 Assets Privately-held investments — trading Commercial mortgage loans $ 211.5 $ 215.7 $ — $ — $ (113.1) $ (2.0) $ 312.1 $ (0.5) Middle market loans and other private debt 65.3 61.8 — — (19.3) (0.9) 106.9 (2.1) Asset-backed securities 26.7 54.5 — — (12.6) (1.8) 66.8 (2.0) Global corporate securities — 15.1 — — — (0.1) 15.0 — Equity securities 3.6 5.5 — — (2.4) (0.1) 6.6 — Short-term investments — 25.6 — — — — 25.6 — Total Level 3 assets $ 307.1 $ 378.2 $ — $ — $ (147.4) $ (4.9) $ 533.0 $ (4.6) Liabilities Derivative liabilities — loss portfolio transfer $ — $ (17.2) $ — $ — $ — $ (14.5) $ (31.7) $ — Total Level 3 liabilities $ — $ (17.2) $ — $ — $ — $ (14.5) $ (31.7) $ — ______________ (1) Increases/(decreases) in the fair value of privately-held investments - trading are included in realized and unrealized investment losses in the consolidated statements of operations and other comprehensive (loss). Increases/(decreases) in the fair value of derivative liabilities - loss portfolio transfer are included within change in fair value of derivatives in the consolidated statements of operations and other comprehensive (loss). (2) Increases/(decreases) in the fair value of privately-held investments - available for sale are included in other comprehensive income (“OCI”). Valuation of Fixed Income Securities . The Company’s fixed income securities are classified as either available for sale or trading and are reported at fair value. As at December 31, 2023 and December 31, 2022, the Company’s fixed income securities were valued by pricing services or broker-dealers using standard market conventions. The market conventions utilize market quotations, market transactions in comparable instruments and various relationships between instruments including, but not limited to, yield to maturity, dollar prices and spread prices in determining value. Independent Pricing Services. The underlying methodology used to determine the fair value of securities in the Company’s available for sale and trading portfolios is by the pricing services. Pricing services will gather observable pricing inputs from multiple external sources, including buy and sell-side contacts and broker-dealers, in order to develop their internal prices. Pricing services provide pricing for less complex, liquid securities based on market quotations in active markets. Pricing services supply prices for a broad range of securities including those for actively traded securities, such as Treasury and other Government securities, in addition to those that trade less frequently or where valuation includes reference to credit spreads, pay down and pre-pay features and other observable inputs. These securities include Government agency, municipals, corporate and asset-backed securities. For securities that may trade less frequently or do not trade on a listed exchange, these pricing services may use matrix pricing consisting of observable market inputs to estimate the fair value of a security. These observable market inputs include reported trades, benchmark yields, broker-dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers, reference data, and industry and economic factors. Additionally, pricing services may use a valuation model such as an option adjusted spread model commonly used for estimating fair values of mortgage-backed and asset-backed securities. The Company does not derive dollar prices using an index as a pricing input for any individual security. Broker-Dealers. The Company obtains quotes from broker-dealers who are active in the corresponding markets when prices are unavailable from independent pricing services or index providers. Generally, broker-dealers value securities through their trading desks based on observable market inputs. Their pricing methodologies include mapping securities based on trade data, bids or offers, observed spreads and performance of newly issued securities. They may also establish pricing through observing secondary trading of similar securities. Quotes from broker-dealers are non-binding. The Company obtains prices for all of its fixed income investment securities via its third-party accounting service provider, and in the majority of cases receiving a number of quotes so as to obtain the most comprehensive information available to determine a security’s fair value. A single valuation is applied to each security based on the vendor hierarchy maintained by the Company’s third-party accounting service provider. As at December 31, 2023, the Company obtained an average of 2.9 quotes per fixed income investment compared to 2.9 quotes at December 31, 2022. The Company, in conjunction with its third-party accounting service provider, obtains an understanding of the methods, models and inputs used by the third-party pricing service and index providers to assess the ongoing appropriateness of vendors’ prices. The Company and its third-party accounting service provider also have controls in place to validate that amounts provided represent fair values. Processes to validate and review pricing include, but are not limited to: • quantitative analysis (e.g., comparing the quarterly return for each managed portfolio to its target benchmark, with significant differences identified and investigated); • comparison of market values obtained from pricing services and broker-dealers against alternative price sources for each security where further investigation is completed when significant differences exist for pricing of individual securities between pricing sources; • initial and ongoing evaluation of methodologies used by outside parties to calculate fair value; and • comparison of the fair value estimates to the Company’s knowledge of the current market. Prices obtained from pricing services and broker-dealers are not adjusted by us; however, prices provided by a pricing service, or broker-dealer in certain instances may be challenged based on market or information available from internal sources, including those available to the Company’s third-party investment accounting service provider. Subsequent to any challenge, revisions made by the pricing service or broker-dealer to the quotes are supplied to the Company’s investment accounting service provider. Management reviews the vendor hierarchy maintained by the Company’s third-party accounting service provider in order to determine which price source provides the most appropriate fair value (i.e., a price obtained from a pricing service with more seniority in the hierarchy will be used over a less senior one in all cases). The hierarchy level assigned to each security in the Company’s available for sale and trading portfolios is based upon its assessment of the transparency and reliability of the inputs used in the valuation as of the measurement date. The hierarchy of pricing services is determined using various qualitative and quantitative points arising from reviews of the vendors conducted by the Company’s third-party accounting service provider. Vendor reviews include annual due diligence meetings with index providers and pricing services vendors covering valuation methodology, operational walkthroughs and legal and compliance updates. Fixed Income Securities . Fixed income securities are traded on the over-the-counter (“OTC”) market based on prices provided by one or more market makers in each security. Securities such as U.S. Government, U.S. Agency, Non-U.S. Government and investment grade corporate bonds have multiple market makers in addition to readily observable market value indicators such as expected credit spread, except for Treasury securities, over the yield curve. The Company uses a variety of pricing sources to value fixed income securities including those securities that have pay down/prepay features such as mortgage-backed securities and asset-backed securities in order to ensure fair and accurate pricing. The fair value estimates for the investment grade securities in the Company’s portfolio do not use significant unobservable inputs or modeling techniques. U.S. Government and Agency Securities. U.S. government and agency securities consist primarily of bonds issued by the U.S. Treasury and corporate debt issued by agencies such as the Federal National Mortgage Association (“FNMA”), the Federal Home Loan Mortgage Corporation (“FHLMC”) and the Federal Home Loan Bank. As the fair values of U.S. Treasury securities are based on unadjusted market prices in active markets, they are classified within Level 1. The fair values of U.S. government agency securities are priced using the spread above the risk-free yield curve. As the yields for the risk-free yield curve and the spreads for these securities are observable market inputs, the fair values of U.S. government agency securities are classified within Level 2. Municipal Securities. The Company’s municipal portfolio consist of bonds issued by U.S. domiciled state and municipality entities. The fair value of these securities is determined using spreads obtained from broker-dealers, trade prices and the new issue market which are Level 2 inputs in the fair value hierarchy. Consequently, these securities are classified within Level 2. Non-U.S. Government. The issuers for securities in this category are non-U.S. governments and their agents including, but not limited to, the U.K., Australia, Canada, France and Germany. The fair values of certain non-U.S. government bonds, primarily sourced from international indices, are based on unadjusted market prices in active markets and are therefore classified within Level 1. The remaining non-U.S. government bonds are classified within Level 2 as they are not actively traded. The fair values of the non-U.S. agency securities, again primarily sourced from international indices, are priced using the spread above the risk-free yield curve. As the yields for the risk-free yield curve and the spreads for these securities are observable market inputs, the fair values of non-U.S. agency securities are classified within Level 2. In addition, foreign government securities include a portion of the Emerging Market Debt (“EMD”) portfolio which is also classified within Level 2. Corporate. Corporate securities consist primarily of short-term, medium-term and long-term debt issued by U.S. and foreign corporations covering a variety of industries and are generally priced by index providers and pricing vendors. Some issuers may participate in government programs which guarantee timely payment of principal and interest in the event of a default. The fair values of these securities are generally determined using the spread above the risk-free yield curve. Inputs used in the evaluation of these securities include credit data, interest rate data, market observations and sector news, broker-dealer quotes and trade volumes. In addition, corporate securities include a portion of the EMD portfolio. The Company classifies these securities within Level 2. Mortgage-backed Securities. Residential and commercial mortgage-backed securities consist of bonds issued by the Government National Mortgage Association, the FNMA and the FHLMC. The fair values of these securities are determined through the use of a pricing model (including Option Adjusted Spread) which uses prepayment speeds and spreads to determine the appropriate average life of the mortgage-backed security. These spreads are generally obtained from broker-dealers, trade prices and the new issue market. As the significant inputs used to price mortgage-backed securities are observable market inputs, these securities are classified within Level 2. Asset-backed securities. Asset-backed securities are securities backed by notes or receivables against assets other than real estate. The underlying collateral for the Company’s asset-backed securities consists mainly of student loans, automobile loans and credit card receivables. These securities are primarily priced by index providers and pricing vendors. Inputs to the valuation process include broker-dealer quotes and other available trade information, prepayment speeds, interest rate data and credit spreads. The Company classifies these securities within Level 2. Short-term investments. Short-term investments consist of highly liquid debt securities with a maturity greater than three months but less than one year from the date of purchase. Short-term investments are classified as either trading or available for sale according to the facts and circumstances of the investment held. Short-term investments are valued in a manner similar to the Company’s fixed maturity investments and are classified within Levels 1 and 2. Privately-Held Investments. Privately-held investments are initially valued at cost or transaction value which approximates fair value. In subsequent measurement periods, the fair values of these securities are determined using discounted cash flow models. These models include inputs that are specific to each investment. The inputs used in the fair value measurements include dividend or interest rates and appropriate discount rates. The selection of an appropriate discount rate is judgmental and is the most significant unobservable input used in the valuation of these securities. A significant increase (decrease) in this input in isolation could result in significantly lower (higher) fair value measurement for privately-held investments. In order to assess the reasonableness of the inputs the Company uses in the discounted cash flow models, the Company maintains an understanding of current market conditions, issuer specific information that may impact future cash flows as well as collaboration with independent vendors for most securities to assess the reasonableness of the discount rate being used. Commercial mortgage loans. Commercial mortgage loans consists of investments in properties including apartments, hotels, office and retail buildings, other commercial properties and industrial properties. The commercial mortgage loan portfolio is diversified by property type, geographic region and issuer to reduce risks. Commercial Mortgage Loans are initially valued at cost or transaction value which approximates fair value. In subsequent measurement periods, the fair values of these securities are determined using discounted cash flow models and are classified as Level 3. Middle market loans and other private debt . The middle market loans consist of investments in senior secured loan positions with full covenants, focused on the middle market in both U.S., Europe and the Caribbean. The other private debt consists of debt securities issued to private investment funds. The middle market loan and other private debt portfolio is diversified by industry type, geographic region and issuer to reduce risks. Middle market loans and other private debt are initially valued at cost or transaction value which approximates fair value. In subsequent measurement periods, the fair values of these securities are determined using discounted cash flow models and are classified as Level 3. Asset-backed securities . Asset-backed securities represent interests in underlying pools of diversified referenced assets that are collateralized and backed by future cash flows and these securities are performing. Asset-backed securities are initially valued at cost or transaction value which approximates fair value. In subsequent measurement periods, the fair values of these securities are determined using discounted cash flow models and are classified as Level 3. Global corporate securities. The global corporates portfolio consists of debt securities with a non-U.S. debt issuer. The fair value of these securities are determined by using discounted cash flow models and are classified as Level 3. Short term investments - privately-held. Short-term investments which are classified as privately-held consist of debt securities with a maturity greater than three months but less than one year from the debt of purchase. Short-term investments are initially valued at cost or transaction value which approximates fair value. In subsequent measurement periods, the fair values of these securities are determined using discounted cash flow models and are classified as Level 3. The following table summarizes the quantitative inputs and assumptions used for financial assets and liabilities investments categorized as Level 3 under the fair value hierarchy as at December 31, 2023: At December 31, 2023 Fair Value Valuation Techniques Unobservable (U) inputs Ranges Weighted Average ($ in millions) Privately-held investments — Trading Commercial mortgage loans $ 274.9 Discounted cash flow Discount rate 3.8 % – 20.0 % 7.7 % Middle market loans and other private debt 84.8 Discounted cash flow Discount rate 7.7 % 18.5 % 11.0 % Asset-backed securities 82.9 Discounted cash flow Discount rate 5.9 % – 9.7 % 7.0 % Global corporate securities 14.4 Discounted cash flow Discount rate 6.6 % – 6.6 % 6.6 % Short-term investments 18.0 Discounted cash flow Discount rate 9.3 % 18.5 % 9.3 % Privately-held investments — Available for sale Asset-backed securities 14.9 Transaction value n/a n/a n/a n/a Total $ 489.9 Catastrophe Bonds. Catastrophe bonds are variable rate fixed income instruments with redemption values adjusted based on the occurrence of a covered event, usually windstorms and earthquakes. Catastrophe bonds are classified as trading and reported at fair value. Catastrophe bonds are priced using an average of multiple broker-dealer quotes and as such, are considered Level 2. Foreign Exchange Contracts. The foreign exchange contracts which the Company uses to mitigate currency risk are characterized as OTC due to their customized nature and the fact that they do not trade on a major exchange. These instruments trade in a very deep liquid market, providing substantial price transparency and accordingly are classified as Level 2. Derivative Liabilities - Loss Portfolio Transfer. The LPT embedded derivative is valued using the Black-Scholes model. The two primary inputs of this model are expected claim settlement patterns and expected return of the investment portfolio above a fixed minimum rate over the specified time horizon. The expected claim settlement pattern is determined on an actuarial basis for the cohort of business within scope of the LPT and is consistent with the patterns used in the valuation of technical provisions. The expected return of the investment portfolio, above a fixed minimum rate, directly impacts on the LPT derivative valuation and is subject to changes in the market conditions. In order to assess the reasonableness of the inputs, the Company updates the expected claim settlement patterns on a regular basis while maintaining an understanding of the current market conditions. Other Investments. The Company’s other investments represent primarily our investments in investment funds operating strategies across real estate, infrastructure and direct lending. Adjustments to the fair values are made based on the net asset value of the investments. The net valuation criteria established by the manager of such investments are established in accordance with the governing documents and the asset manager’s valuation guidelines, which include: the discounted cash flows method and the performance multiple approach, which uses a multiple derived from market data of comparable companies or assets to produce operating performance metrics. Alternative valuation methodologies may be employed for investments with unusual characteristics.

Reinsurance

Reinsurance	12 Months Ended
	Dec. 31, 2023
Reinsurance Disclosures [Abstract]
Reinsurance	Reinsurance The Company purchases retrocession and reinsurance to limit and diversify the Company’s risk exposure and to increase its own insurance and reinsurance underwriting capacity. These agreements provide for recovery of losses and loss adjustment expenses from reinsurers. The Company remains liable to the extent that reinsurers do not meet their obligations under these agreements. In line with its risk management objectives, the Company evaluates the financial condition of its reinsurers and monitors concentrations of credit risk. Balances pertaining to reinsurance transactions are reported “gross” on the consolidated balance sheet, meaning that reinsurance recoverable on unpaid losses and ceded unearned premiums are not deducted from insurance reserves but are recorded as assets. For more information on reinsurance recoverables, refer to Note 22, “Concentrations of Credit Risk — Reinsurance recoverables” and Note 10, “Reserve for Losses and Loss Adjustment Expenses” of these consolidated financial statements. The effect of assumed and ceded reinsurance on premiums written, premiums earned and losses and loss adjustment expenses for the twelve months ended December 31, 2023, 2022 and 2021 was as follows: Twelve Months Ended December 31, 2023 2022 2021 ($ in millions) Premiums written: Insurance $ 2,446.6 $ 2,531.7 $ 2,341.4 Reinsurance 1,521.0 1,807.0 1,597.0 Ceded (1,385.7) (1,442.7) (1,350.7) Net written premiums $ 2,581.9 $ 2,896.0 $ 2,587.7 Premiums earned: Insurance $ 2,444.8 $ 2,370.8 $ 2,139.1 Reinsurance 1,562.0 1,617.2 1,479.2 Ceded (1,392.3) (1,299.3) (1,207.8) Net earned premiums $ 2,614.5 $ 2,688.7 $ 2,410.5 Losses and loss adjustment expenses: Insurance $ 1,644.5 $ 1,574.2 $ 1,499.8 Reinsurance 707.2 939.5 1,000.6 Ceded (798.7) (833.7) (807.1) Losses and loss adjustment expenses $ 1,553.0 $ 1,680.0 $ 1,693.3 Current expected credit loss model (“CECL”) . As at December 31, 2023, the Company’s allowance for expected credit losses was $3.7 million (2022 — $3.7 million). For the twelve months ended December 31, 2023 there was no change in the CECL allowance on reinsurance recoverables (2022 — $0.4 million increase, 2021 — $0.5 million decrease).

Derivative Contracts

Derivative Contracts	12 Months Ended
	Dec. 31, 2023
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Contracts	Derivative Contracts The following table summarizes information on the location and amounts of derivative fair values on the consolidated balance sheet as at December 31, 2023 and 2022: As at December 31, 2023 As at December 31, 2022 Derivatives Not Designated as Hedging Instruments Balance Sheet Location Notional Fair Notional Fair ($ in millions) ($ in millions) Foreign Exchange Contracts (1) Derivative assets $ 1,262.1 $ 31.4 $ 945.8 $ 41.9 Foreign Exchange Contracts Derivative liabilities $ 540.8 $ (9.3) $ 729.5 $ (3.2) Loss Portfolio Transfer Liability - Embedded Derivative (2) Derivative liabilities — $ (16.5) — (31.7) ________________ (1) Fair value is net of $3.4 million of cash collateral (December 31, 2022 — $3.7 million). (2) The LPT contains an embedded derivative within the contract in relation to the variable interest crediting rate. As at December 31, 2023 As at December 31, 2022 Derivatives Designated as Cash Flow Hedges Under ASC 815 Balance Sheet Location Notional Fair Notional Fair ($ in millions) ($ in millions) Foreign Exchange Contracts Derivative assets $ 76.9 $ 0.3 $ 109.7 $ 14.3 The following table provides the unrealized and realized gains/(losses) recorded in the consolidated statements of operations and other comprehensive income for derivatives that are not designated or designated as hedging instruments under ASC 815 — “Derivatives and Hedging” for the twelve months ended December 31, 2023 and 2022: Amount of (Loss)/Gain Recognized on Derivatives For the Twelve Months Ended Location of Gain/(Loss) December 31, 2023 December 31, 2022 Derivatives not designated as hedges ($ in millions) Foreign Exchange Contracts Change in Fair Value of Derivatives 10.9 (66.0) Loss Portfolio Transfer Liability - Embedded Derivative Change in Fair Value of Derivatives 15.2 (14.5) Derivatives designated as cash flow hedges Foreign Exchange Contracts General, administrative and corporate expenses in consolidated statement of operations (8.1) 5.9 Foreign Exchange Contracts Net change from current period hedged transactions in other comprehensive income (14.0) 15.4 Foreign Exchange Contracts. The Company uses foreign exchange contracts to manage foreign currency risk associated with our operating expenses but also foreign exchange risk associated with net assets or liabilities in currencies other than the U.S. dollar. A foreign exchange contract involves an obligation to purchase or sell a specified currency at a future date at a price set at the time of the contract. Foreign exchange contracts will not eliminate fluctuations in the value of the Company’s assets and liabilities denominated in foreign currencies but rather allow it to establish a rate of exchange for a future point in time. As at December 31, 2023, the Company held foreign exchange contracts that were not designated as hedges under ASC 815 with an aggregate nominal amount of $1,802.9 million (2022 — $1,675.3 million). The foreign exchange contracts are recorded as derivative assets or derivative liabilities in the consolidated balance sheet with changes recorded as a change in fair value of derivatives in the consolidated statement of operations. For the twelve months ended December 31, 2023, the impact of foreign exchange contracts on net income was a gain of $10.9 million (December 31, 2022 — loss of $66.0 million). As at December 31, 2023, the Company held foreign exchange contracts that were designated as cash flow hedges under ASC 815 with an aggregate notional amount of $76.9 million (2022 — $109.7 million). The foreign exchange contracts are recorded as derivative assets in the consolidated balance sheet with the changes in fair value recorded in other comprehensive income. For the twelve months ended December 31, 2023 the company recognized a loss of $14.0 million (December 31, 2022 — gain of $15.4 million) in other comprehensive income. As the foreign exchange contracts settle, the realized gain or loss is reclassified from other comprehensive income into general, administration and corporate expenses in the consolidated statement of operations. For the twelve months ended December 31, 2023, the amount recognized within general, administration and corporate expenses for settled foreign exchange contracts was a realized loss of $8.1 million (December 31, 2022 — gain of $5.9 million). The Company estimates that $0.3 million of the existing gains as at December 31, 2023 is expected to be reclassified into earnings within the next 12 months. Embedded derivative on loss portfolio contract. The loss portfolio transfer contract includes a funds withheld arrangement that provides returns to the reinsurer based on Aspen’s investment performance, guaranteeing a minimum of 1.75% return. Such funds withheld arrangements are examples of embedded derivatives and therefore this instrument is accounted for as an option-based derivative. For the twelve months ended December 31, 2023, the amount recognized as a change in fair value of derivatives in the consolidated statement of operations is a gain of $15.2 million (December 31, 2022 — loss of $14.5 million).

Deferred Policy Acquisition Cos

Deferred Policy Acquisition Costs	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Insurance [Abstract]
Deferred Policy Acquisition Costs	Twelve Months Ended December 31, 2023 Twelve Months Ended December 31, 2022 ($ in millions) Balance at the beginning of the period $ 319.0 $ 290.8 Acquisition costs deferred 357.4 460.0 Amortization of deferred acquisition costs (380.2) (431.8) Balance at the end of the period $ 296.2 $ 319.0	Deferred Policy Acquisition Costs The following table represents a reconciliation of beginning and ending deferred acquisition costs for the twelve months ended December 31, 2023 and 2022: Twelve Months Ended December 31, 2023 Twelve Months Ended December 31, 2022 ($ in millions) Balance at the beginning of the period $ 319.0 $ 290.8 Acquisition costs deferred 357.4 460.0 Amortization of deferred acquisition costs (380.2) (431.8) Balance at the end of the period $ 296.2 $ 319.0

Reserves for Losses and Adjustm

Reserves for Losses and Adjustment Expenses	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Insurance [Abstract]
Reserves for Losses and Adjustment Expenses	As at December 31, 2023 2022 2021 ($ in millions) Reserve for losses and LAE at the start of the year $ 7,710.9 $ 7,611.8 $ 7,165.3 Less reinsurance recoverable (4,897.7) (3,298.1) (3,195.2) Net reserve for losses and LAE at the start of the year 2,813.2 4,313.7 3,970.1 Net loss and LAE expenses disposed (1) — (1,840.1) — Movement in net reserve for losses and LAE for claims incurred: Current year 1,492.2 1,651.9 1,648.2 Prior years 60.8 28.1 45.1 Total incurred 1,553.0 1,680.0 1,693.3 Net Losses and LAE payments for claims incurred: Current year (161.1) (192.7) (729.1) Prior years (1,011.9) (1,098.4) (580.7) Total paid (1,173.0) (1,291.1) (1,309.8) Foreign exchange (gains)/losses 39.6 (49.3) (39.9) Net reserve for losses and LAE at the end of the year 3,232.8 2,813.2 4,313.7 Plus reinsurance recoverable on unpaid losses at the end of the year 4,577.8 4,897.7 3,298.1 Reserve for losses and LAE at the end of the year $ 7,810.6 $ 7,710.9 $ 7,611.8 ________________ (1) Net loss and LAE expenses disposed of $1,840.1 million represent the net loss reserves as at May 20, 2022 (“Closing Date”) for losses in relation to 2019 and prior accident years, in addition to the $770.0 million of ceded reserves under the previous ADC agreement, recognizing a total recoverable of $2,610.1 million. These reserves were rolled forward from the initial effective date of September 30, 2021, at which time the net losses reserves were $3,120.0 million. As at December 31, 2023, the total amount recoverable from Enstar under the LPT was $1,627.4 million (December 31, 2022 — $2,132.0 million) which includes claims paid and reserve development since the Closing Date. Property Insurance Lines As at December 31, 2023 Incurred Claims, IBNR and Loss Adjustment Expenses, Net of Reinsurance Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims Accident Year For the Years Ended December 31, Unaudited Prior Years 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 $ (in millions) 2014 164.3 156.3 133.5 134.1 133.3 131.8 131.2 125.8 132.8 132.8 — 10,037 2015 237.3 203.0 197.7 200.0 200.5 197.5 181.9 193.4 193.4 — 11,624 2016 236.8 247.7 242.6 244.0 245.3 234.2 233.4 233.4 — 10,844 2017 293.8 256.8 250.0 251.3 273.0 261.6 261.6 — 9,805 2018 200.7 203.0 186.8 196.3 180.2 180.2 — 8,395 2019 125.3 129.2 102.8 109.0 109.0 — 6,944 2020 203.2 198.3 208.0 213.5 25.9 7,689 2021 207.4 202.0 195.5 16.1 6,810 2022 167.1 174.2 22.3 5,702 2023 152.1 81.7 3,297 Total $ 1,845.7 Property Insurance Lines Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Unaudited Prior Years Accident Year 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 ($ in millions) 2014 40.2 86.0 113.5 123.1 127.1 128.5 129.8 130.9 132.8 132.8 2015 56.9 141.0 168.4 177.5 193.8 192.4 193.4 193.4 193.4 2016 66.5 167.8 200.2 221.9 231.1 234.9 233.4 233.4 2017 96.0 187.3 220.0 240.9 234.8 261.6 261.6 2018 61.4 158.1 180.4 174.1 180.2 180.2 2019 48.7 90.5 98.4 109.0 109.0 2020 61.0 124.1 151.8 169.6 2021 58.7 119.7 151.2 2022 41.1 113.7 2023 30.0 Total $ 1,574.9 All outstanding liabilities for 2014 and subsequent years, net of reinsurance $ 270.8 All outstanding liabilities before 2014, net of reinsurance — Liabilities for claims and claim adjustment expenses, net of reinsurance $ 270.8 Casualty Insurance Lines As at December 31, 2023 Incurred Claims, IBNR and Loss Adjustment Expenses, Net of Reinsurance Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims Accident Year For the Years Ended December 31, Unaudited Prior Years 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 $ (in millions) 2014 142.9 125.3 137.1 127.1 134.2 138.1 135.1 141.4 107.0 107.0 — 3,962 2015 201.3 221.3 183.9 201.3 234.0 232.3 257.9 180.5 180.5 — 4,811 2016 215.0 186.0 181.3 187.8 198.9 243.5 124.3 124.3 — 4,815 2017 179.4 172.9 176.7 194.7 215.9 61.4 61.4 — 5,470 2018 121.5 124.5 134.9 164.7 43.7 43.7 — 5,455 2019 124.1 146.5 153.5 48.6 48.6 — 5,156 2020 132.9 141.6 140.4 146.6 50.2 3,753 2021 173.5 188.6 197.8 99.4 3,343 2022 204.7 215.1 148.3 3,050 2023 224.9 173.0 2,465 Total $ 1,349.9 Casualty Insurance Lines Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Unaudited Prior Years Accident Year 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 ($ in millions) 2014 2.6 13.1 32.2 58.8 71.9 95.6 108.2 106.8 107.0 107.0 2015 3.1 16.8 56.0 91.8 137.2 166.2 180.5 180.5 180.5 2016 4.1 22.5 39.6 81.5 108.4 123.3 124.3 124.3 2017 3.5 22.7 52.2 95.8 89.4 61.4 61.4 2018 3.1 27.7 42.7 58.1 43.7 43.7 2019 6.3 17.6 64.1 48.6 48.6 2020 — 9.3 36.3 61.6 2021 3.1 23.5 53.8 2022 8.90 5.6 2023 26.9 Total $ 713.4 All outstanding liabilities for 2014 and subsequent years, net of reinsurance $ 636.5 All outstanding liabilities before 2014, net of reinsurance — Liabilities for claims and claim adjustment expenses, net of reinsurance $ 636.5 Marine, Aviation and Energy Insurance Lines As at December 31, 2023 Incurred Claims, IBNR and Loss Adjustment Expenses, Net of Reinsurance Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims For the Years Ended December 31, Unaudited Prior Years Accident Year 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 $ (in millions) 2014 308.8 312.3 297.4 309.2 304.8 311.7 301.0 312.4 271.3 271.3 — 4,056 2015 295.4 297.6 280.1 284.8 308.3 311.1 318.3 267.3 267.3 — 4,067 2016 259.5 229.5 228.3 228.7 218.5 220.3 197.9 197.9 — 4,422 2017 209.6 200.2 206.7 214.2 225.6 141.4 141.4 — 6,078 2018 170.4 207.4 208.3 234.8 151.0 151.0 — 5,183 2019 145.6 153.0 123.5 102.4 102.4 — 3,691 2020 110.2 111.2 125.7 126.6 9.6 3,847 2021 93.1 96.2 95.7 10.1 4,787 2022 108.0 106.1 31.2 5,428 2023 117.4 72.6 2,703 Total $ 1,577.1 Marine, Aviation and Energy Insurance Lines Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Unaudited Prior Years Accident Year 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 ($ in millions) 2014 53.3 116.3 188.3 208.9 231.6 249.8 262.0 269.0 271.3 271.3 2015 44.7 122.7 173.3 193.0 220.9 256.5 268.4 267.3 267.3 2016 30.9 82.3 142.1 163.8 190.4 193.3 197.9 197.9 2017 40.1 97.4 140.1 168.2 149.8 141.4 141.4 2018 26.7 104.7 133.0 151.0 151.0 151.0 2019 33.5 72.5 89.6 102.4 102.4 2020 28.5 66.5 88.7 101.2 2021 23.5 52.3 64.7 2022 24.9 57.6 2023 27.8 Total $ 1,382.6 All outstanding liabilities for 2014 and subsequent years, net of reinsurance $ 194.5 All outstanding liabilities before 2014, net of reinsurance — Liabilities for claims and claim adjustment expenses, net of reinsurance $ 194.5 Financial and Professional Insurance Lines As at December 31, 2023 Incurred Claims, IBNR and Loss Adjustment Expenses, Net of Reinsurance Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims For the Years Ended December 31, Unaudited Prior Years Accident Year 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 $ (in millions) 2014 134.3 130.1 128.6 119.0 130.2 119.2 120.3 121.7 98.8 98.8 — 794 2015 173.3 174.8 184.6 188.7 189.8 184.6 196.7 145.9 145.9 — 1,083 2016 190.1 210.9 215.4 201.2 184.6 186.1 134.3 134.3 — 1,241 2017 205.5 181.6 186.5 187.1 209.1 136.0 136.0 — 1,751 2018 155.7 171.6 153.2 161.9 111.2 111.2 — 4,636 2019 248.2 261.1 238.9 132.7 132.7 — 23,826 2020 348.1 347.9 338.2 352.3 87.4 106,054 2021 286.2 304.8 296.2 128.4 34,929 2022 317.0 302.4 184.6 3,422 2023 341.6 277.0 3,137 Total $ 2,051.4 Financial and Professional Insurance Lines Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Unaudited Prior Years Accident Year 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 ($ in millions) 2014 2.9 30.5 53.2 71.8 79.2 84.9 91.7 99.3 98.8 98.8 2015 13.7 43.3 69.9 89.1 109.5 138.4 150.9 145.9 145.9 2016 15.0 71.0 101.5 129.6 125.5 130.1 134.3 134.3 2017 27.1 51.2 83.2 116.8 134.8 136.0 136.0 2018 19.1 73.4 99.0 111.2 111.2 111.2 2019 27.1 86.6 121.1 132.7 132.7 2020 47.6 121.2 174.7 226.2 2021 43.2 90.4 131.7 2022 17.8 75.5 2023 21.5 Total $ 1,213.8 All outstanding liabilities for 2014 and subsequent years, net of reinsurance $ 837.6 All outstanding liabilities before 2014, net of reinsurance — Liabilities for claims and claim adjustment expenses, net of reinsurance $ 837.6 Reconciliation of Incurred and Paid Claims Development to total Reserve for Losses and LAE As at December 31, 2023 As at December 31, 2022 ($ in millions) Net outstanding liabilities: Insurance lines - Property insurance lines 270.8 259.0 - Casualty insurance lines 636.5 457.3 - Marine, aviation and energy insurance lines 194.5 163.7 - Financial and professional insurance lines 837.6 671.9 Total insurance lines 1,939.4 1,551.9 Reinsurance lines - Property catastrophe and other property reinsurance 562.3 677.9 - Casualty reinsurance 733.6 571.6 - Specialty reinsurance 394.3 350.2 Total reinsurance lines 1,690.2 1,599.7 Net loss and LAE 3,629.6 3,151.6 Reinsurance recoverable on unpaid losses: Insurance lines 2,821.6 2,907.8 Reinsurance lines 1,756.2 1,989.9 Total reinsurance recoverable on unpaid losses 4,577.8 4,897.7 Deferred gain on retroactive contracts 27.3 42.7 Unallocated claims incurred 47.9 41.4 Other reinsurance balances recoverable (1) (489.1) (429.3) Carbon syndicate reserves 16.7 5.2 Other 0.4 1.6 (396.8) (338.4) Reserve for losses and LAE at the end of the year 7,810.6 7,710.9 ________________ (1)	Reserve for Losses and Loss Adjustment Expenses The following table represents a reconciliation of beginning and ending consolidated reserve for losses and loss adjustment expenses for the twelve months ended December 31, 2023, 2022 and 2021: As at December 31, 2023 2022 2021 ($ in millions) Reserve for losses and LAE at the start of the year $ 7,710.9 $ 7,611.8 $ 7,165.3 Less reinsurance recoverable (4,897.7) (3,298.1) (3,195.2) Net reserve for losses and LAE at the start of the year 2,813.2 4,313.7 3,970.1 Net loss and LAE expenses disposed (1) — (1,840.1) — Movement in net reserve for losses and LAE for claims incurred: Current year 1,492.2 1,651.9 1,648.2 Prior years 60.8 28.1 45.1 Total incurred 1,553.0 1,680.0 1,693.3 Net Losses and LAE payments for claims incurred: Current year (161.1) (192.7) (729.1) Prior years (1,011.9) (1,098.4) (580.7) Total paid (1,173.0) (1,291.1) (1,309.8) Foreign exchange (gains)/losses 39.6 (49.3) (39.9) Net reserve for losses and LAE at the end of the year 3,232.8 2,813.2 4,313.7 Plus reinsurance recoverable on unpaid losses at the end of the year 4,577.8 4,897.7 3,298.1 Reserve for losses and LAE at the end of the year $ 7,810.6 $ 7,710.9 $ 7,611.8 ________________ (1) Net loss and LAE expenses disposed of $1,840.1 million represent the net loss reserves as at May 20, 2022 (“Closing Date”) for losses in relation to 2019 and prior accident years, in addition to the $770.0 million of ceded reserves under the previous ADC agreement, recognizing a total recoverable of $2,610.1 million. These reserves were rolled forward from the initial effective date of September 30, 2021, at which time the net losses reserves were $3,120.0 million. As at December 31, 2023, the total amount recoverable from Enstar under the LPT was $1,627.4 million (December 31, 2022 — $2,132.0 million) which includes claims paid and reserve development since the Closing Date. For the twelve months ended December 31, 2023, there was an increase of $60.8 million in the Company’s estimate of the ultimate claims to be paid in respect of prior accident years compared to an increase of $28.1 million for the twelve months ended December 31, 2022. The following tables show an analysis of incurred claims and allocated loss adjustment expenses, net of reinsurance and cumulative paid claims and allocated claim adjustment expenses, net of reinsurance for each of the years ended December 31, 2014 through 2023. Under the LPT agreement, the Company has reinsured net losses incurred on all accident years 2019 and prior. This has resulted in IBNR in the loss development triangles for 2019 and prior to be Nil. The loss development triangles are derived from all business written by the Company as although a limited number of contracts are written which have durations of greater than one year the contracts do not meet the definition of a long duration contract. All amounts included in the following tables related to transactions denominated in a foreign currency have been translated into U.S. Dollars using the exchange rates in effect at December 31, 2023. The Company has chosen to disaggregate the business in its Insurance segment, for the purposes of these loss development triangles as: Property; Casualty; Marine Aviation and Energy; and Financial and Professional insurance lines. The Company considers that this presentation of its Insurance lines loss development triangles more precisely reflects meaningful trending information. The Company presents its loss development triangles for the Reinsurance segment in line with the reportable reinsurance lines: Property Catastrophe and Other Property; Casualty; and Specialty. Property Insurance Lines As at December 31, 2023 Incurred Claims, IBNR and Loss Adjustment Expenses, Net of Reinsurance Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims Accident Year For the Years Ended December 31, Unaudited Prior Years 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 $ (in millions) 2014 164.3 156.3 133.5 134.1 133.3 131.8 131.2 125.8 132.8 132.8 — 10,037 2015 237.3 203.0 197.7 200.0 200.5 197.5 181.9 193.4 193.4 — 11,624 2016 236.8 247.7 242.6 244.0 245.3 234.2 233.4 233.4 — 10,844 2017 293.8 256.8 250.0 251.3 273.0 261.6 261.6 — 9,805 2018 200.7 203.0 186.8 196.3 180.2 180.2 — 8,395 2019 125.3 129.2 102.8 109.0 109.0 — 6,944 2020 203.2 198.3 208.0 213.5 25.9 7,689 2021 207.4 202.0 195.5 16.1 6,810 2022 167.1 174.2 22.3 5,702 2023 152.1 81.7 3,297 Total $ 1,845.7 Property Insurance Lines Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Unaudited Prior Years Accident Year 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 ($ in millions) 2014 40.2 86.0 113.5 123.1 127.1 128.5 129.8 130.9 132.8 132.8 2015 56.9 141.0 168.4 177.5 193.8 192.4 193.4 193.4 193.4 2016 66.5 167.8 200.2 221.9 231.1 234.9 233.4 233.4 2017 96.0 187.3 220.0 240.9 234.8 261.6 261.6 2018 61.4 158.1 180.4 174.1 180.2 180.2 2019 48.7 90.5 98.4 109.0 109.0 2020 61.0 124.1 151.8 169.6 2021 58.7 119.7 151.2 2022 41.1 113.7 2023 30.0 Total $ 1,574.9 All outstanding liabilities for 2014 and subsequent years, net of reinsurance $ 270.8 All outstanding liabilities before 2014, net of reinsurance — Liabilities for claims and claim adjustment expenses, net of reinsurance $ 270.8 Casualty Insurance Lines As at December 31, 2023 Incurred Claims, IBNR and Loss Adjustment Expenses, Net of Reinsurance Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims Accident Year For the Years Ended December 31, Unaudited Prior Years 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 $ (in millions) 2014 142.9 125.3 137.1 127.1 134.2 138.1 135.1 141.4 107.0 107.0 — 3,962 2015 201.3 221.3 183.9 201.3 234.0 232.3 257.9 180.5 180.5 — 4,811 2016 215.0 186.0 181.3 187.8 198.9 243.5 124.3 124.3 — 4,815 2017 179.4 172.9 176.7 194.7 215.9 61.4 61.4 — 5,470 2018 121.5 124.5 134.9 164.7 43.7 43.7 — 5,455 2019 124.1 146.5 153.5 48.6 48.6 — 5,156 2020 132.9 141.6 140.4 146.6 50.2 3,753 2021 173.5 188.6 197.8 99.4 3,343 2022 204.7 215.1 148.3 3,050 2023 224.9 173.0 2,465 Total $ 1,349.9 Casualty Insurance Lines Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Unaudited Prior Years Accident Year 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 ($ in millions) 2014 2.6 13.1 32.2 58.8 71.9 95.6 108.2 106.8 107.0 107.0 2015 3.1 16.8 56.0 91.8 137.2 166.2 180.5 180.5 180.5 2016 4.1 22.5 39.6 81.5 108.4 123.3 124.3 124.3 2017 3.5 22.7 52.2 95.8 89.4 61.4 61.4 2018 3.1 27.7 42.7 58.1 43.7 43.7 2019 6.3 17.6 64.1 48.6 48.6 2020 — 9.3 36.3 61.6 2021 3.1 23.5 53.8 2022 8.90 5.6 2023 26.9 Total $ 713.4 All outstanding liabilities for 2014 and subsequent years, net of reinsurance $ 636.5 All outstanding liabilities before 2014, net of reinsurance — Liabilities for claims and claim adjustment expenses, net of reinsurance $ 636.5 Marine, Aviation and Energy Insurance Lines As at December 31, 2023 Incurred Claims, IBNR and Loss Adjustment Expenses, Net of Reinsurance Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims For the Years Ended December 31, Unaudited Prior Years Accident Year 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 $ (in millions) 2014 308.8 312.3 297.4 309.2 304.8 311.7 301.0 312.4 271.3 271.3 — 4,056 2015 295.4 297.6 280.1 284.8 308.3 311.1 318.3 267.3 267.3 — 4,067 2016 259.5 229.5 228.3 228.7 218.5 220.3 197.9 197.9 — 4,422 2017 209.6 200.2 206.7 214.2 225.6 141.4 141.4 — 6,078 2018 170.4 207.4 208.3 234.8 151.0 151.0 — 5,183 2019 145.6 153.0 123.5 102.4 102.4 — 3,691 2020 110.2 111.2 125.7 126.6 9.6 3,847 2021 93.1 96.2 95.7 10.1 4,787 2022 108.0 106.1 31.2 5,428 2023 117.4 72.6 2,703 Total $ 1,577.1 Marine, Aviation and Energy Insurance Lines Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Unaudited Prior Years Accident Year 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 ($ in millions) 2014 53.3 116.3 188.3 208.9 231.6 249.8 262.0 269.0 271.3 271.3 2015 44.7 122.7 173.3 193.0 220.9 256.5 268.4 267.3 267.3 2016 30.9 82.3 142.1 163.8 190.4 193.3 197.9 197.9 2017 40.1 97.4 140.1 168.2 149.8 141.4 141.4 2018 26.7 104.7 133.0 151.0 151.0 151.0 2019 33.5 72.5 89.6 102.4 102.4 2020 28.5 66.5 88.7 101.2 2021 23.5 52.3 64.7 2022 24.9 57.6 2023 27.8 Total $ 1,382.6 All outstanding liabilities for 2014 and subsequent years, net of reinsurance $ 194.5 All outstanding liabilities before 2014, net of reinsurance — Liabilities for claims and claim adjustment expenses, net of reinsurance $ 194.5 Financial and Professional Insurance Lines As at December 31, 2023 Incurred Claims, IBNR and Loss Adjustment Expenses, Net of Reinsurance Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims For the Years Ended December 31, Unaudited Prior Years Accident Year 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 $ (in millions) 2014 134.3 130.1 128.6 119.0 130.2 119.2 120.3 121.7 98.8 98.8 — 794 2015 173.3 174.8 184.6 188.7 189.8 184.6 196.7 145.9 145.9 — 1,083 2016 190.1 210.9 215.4 201.2 184.6 186.1 134.3 134.3 — 1,241 2017 205.5 181.6 186.5 187.1 209.1 136.0 136.0 — 1,751 2018 155.7 171.6 153.2 161.9 111.2 111.2 — 4,636 2019 248.2 261.1 238.9 132.7 132.7 — 23,826 2020 348.1 347.9 338.2 352.3 87.4 106,054 2021 286.2 304.8 296.2 128.4 34,929 2022 317.0 302.4 184.6 3,422 2023 341.6 277.0 3,137 Total $ 2,051.4 Financial and Professional Insurance Lines Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Unaudited Prior Years Accident Year 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 ($ in millions) 2014 2.9 30.5 53.2 71.8 79.2 84.9 91.7 99.3 98.8 98.8 2015 13.7 43.3 69.9 89.1 109.5 138.4 150.9 145.9 145.9 2016 15.0 71.0 101.5 129.6 125.5 130.1 134.3 134.3 2017 27.1 51.2 83.2 116.8 134.8 136.0 136.0 2018 19.1 73.4 99.0 111.2 111.2 111.2 2019 27.1 86.6 121.1 132.7 132.7 2020 47.6 121.2 174.7 226.2 2021 43.2 90.4 131.7 2022 17.8 75.5 2023 21.5 Total $ 1,213.8 All outstanding liabilities for 2014 and subsequent years, net of reinsurance $ 837.6 All outstanding liabilities before 2014, net of reinsurance — Liabilities for claims and claim adjustment expenses, net of reinsurance $ 837.6 Property Catastrophe and Other Property Reinsurance As at December 31, 2023 Incurred Claims, IBNR and Loss Adjustment Expenses, Net of Reinsurance Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims For the Years Ended December 31, Unaudited Prior Years Accident Year 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 $ (in millions) 2014 185.8 172.8 157.2 145.7 145.8 141.1 141.1 139.7 137.1 137.1 — 901 2015 211.3 185.1 175.3 154.8 170.3 170.4 176.8 155.3 155.3 — 1,051 2016 266.4 266.3 265.0 243.8 239.2 230.7 228.6 228.6 — 1,303 2017 552.3 531.4 513.1 501.8 573.6 431.8 431.8 — 1,958 2018 318.2 351.9 344.1 534.7 282.1 282.1 — 1,777 2019 228.0 241.5 332.4 162.7 162.7 — 1,401 2020 317.6 403.7 350.7 362.6 (5.5) 1,418 2021 652.9 479.3 498.9 30.7 1,498 2022 393.7 392.5 48.9 1,404 2023 230.8 117.7 753 Total $ 2,882.4 Property Catastrophe and Other Property Reinsurance Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Unaudited Prior Years Accident Year 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 ($ in millions) 2014 36.8 98.7 124.5 133.8 137.3 135.9 137.5 137.4 137.1 137.1 2015 35.8 94.2 125.4 137.8 154.8 157.1 158.6 155.3 155.3 2016 56.3 161.7 202.4 213.2 226.1 231.7 228.6 228.6 2017 123.2 356.1 414.5 438.4 411.7 431.8 431.8 2018 122.4 282.2 286.2 279.5 282.1 282.1 2019 28.1 140.8 167.5 162.7 162.7 2020 41.9 165.7 237.3 312.3 2021 75.0 235.7 364.3 2022 65.2 200.8 2023 45.1 Total $ 2,320.1 All outstanding liabilities for 2014 and subsequent years, net of reinsurance 562.3 All outstanding liabilities before 2014, net of reinsurance — Liabilities for claims and claim adjustment expenses, net of reinsurance $ 562.3 Casualty Reinsurance As at December 31, 2023 Incurred Claims, IBNR and Loss Adjustment Expenses, Net of Reinsurance Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims For the Years Ended December 31, Unaudited Prior Years Accident Year 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 $ (in millions) 2014 203.4 205.8 214.2 207.5 201.0 204.0 199.9 189.8 131.0 131.0 — 1,866 2015 192.5 199.2 208.9 211.4 209.0 205.2 195.2 116.5 116.5 — 2,067 2016 231.1 243.5 243.0 252.9 260.3 251.2 137.1 137.1 — 2,231 2017 242.7 240.4 251.0 250.4 261.6 110.8 110.8 — 2,284 2018 227.2 256.5 264.1 254.3 92.7 92.7 — 2,149 2019 233.5 254.0 244.3 52.9 52.9 — 1,790 2020 253.6 234.7 198.8 181.2 62.0 1,389 2021 206.4 216.3 205.7 93.3 1,325 2022 249.5 250.6 178.3 1,256 2023 271.6 244.9 779 Total $ 1,550.1 Casualty Reinsurance Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Unaudited Prior Years Accident Year 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 ($ in millions) 2014 2.5 13.7 37.6 59.9 86.0 106.8 124.4 131.0 131.0 131.0 2015 3.4 17.8 38.1 65.2 89.0 108.0 116.5 116.5 116.5 2016 9.1 33.3 63.6 95.6 125.6 137.1 137.1 137.1 2017 8.8 30.4 58.8 97.0 110.7 110.8 110.8 2018 7.1 33.4 73.3 92.7 92.7 92.7 2019 9.2 36.4 52.5 52.9 52.9 2020 9.1 27.8 44.3 71.9 2021 7.8 37.3 64.0 2022 9.4 31.1 2023 8.5 Total $ 816.5 All outstanding liabilities for 2014 and subsequent years, net of reinsurance 733.6 All outstanding liabilities before 2014, net of reinsurance — Liabilities for claims and claim adjustment expenses, net of reinsurance $ 733.6 Specialty Reinsurance As at December 31, 2023 Incurred Claims, IBNR and Loss Adjustment Expenses, Net of Reinsurance Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims For the Years Ended December 31, Unaudited Prior Years Accident Year 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 $ (in millions) 2014 150.7 139.0 130.9 122.1 124.9 123.5 119.3 116.4 104.0 104.0 — 618 2015 164.7 168.0 162.7 157.3 155.9 151.4 151.1 132.1 132.1 — 774 2016 237.2 238.3 236.1 228.9 224.0 211.6 188.2 188.2 — 937 2017 377.4 390.4 374.1 363.0 356.4 305.9 305.9 — 1,336 2018 393.8 393.3 391.7 416.2 324.3 324.3 — 1,418 2019 472.6 495.9 398.7 401.1 401.1 — 1,543 2020 414.1 601.3 375.4 379.5 36.7 1,494 2021 157.4 152.5 142.1 41.1 1,356 2022 194.7 194.3 108.9 1,319 2023 142.5 100.4 952 Total $ 2,314.0 Specialty Reinsurance Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Unaudited Prior Years Accident Year 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 ($ in millions) 2014 16.4 55.9 80.6 88.7 99.0 101.9 103.6 104.1 104.0 104.0 2015 17.5 55.8 103.3 120.9 129.9 132.9 132.6 132.1 132.1 2016 58.4 150.3 164.7 182.7 192.9 194.3 188.2 188.2 2017 94.5 238.2 270.0 305.2 306.4 305.9 305.9 2018 27.1 279.6 313.0 324.7 324.3 324.3 2019 273.2 381.0 399.2 401.1 401.1 2020 213.0 270.1 290.9 311.6 2021 28.3 53.4 76.1 2022 26.0 53.8 2023 22.6 Total $ 1,919.7 All outstanding liabilities for 2014 and subsequent years, net of reinsurance 394.3 All outstanding liabilities before 2014, net of reinsurance — Liabilities for claims and claim adjustment expenses, net of reinsurance $ 394.3 Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance (unaudited) Years 1 2 3 4 5 6 7 8 9 10 Property insurance lines 30.6 % 39.0 % 13.7 % 6.3 % 2.7 % 2.4 % 0.2 % 0.3 % 0.7 % — % Casualty insurance lines 4.1 % 18.7 % 33.1 % 24.2 % 2.7 % 1.0 % 5.1 % (0.4) % 0.1 % — % Marine, aviation and energy insurance lines 22.5 % 33.3 % 21.5 % 11.4 % 3.2 % 3.1 % 2.8 % 0.7 % 0.4 % — % Financial and professional insurance lines 12.1 % 28.6 % 20.7 % 16.0 % 5.3 % 6.0 % 4.7 % 1.4 % (0.3) % — % Property catastrophe and other property reinsurance 22.6 % 45.5 % 16.7 % 5.8 % 2.3 % 1.5 % 0.2 % (0.7) % (0.1) % — % Casualty reinsurance 6.0 % 19.0 % 22.4 % 19.3 % 12.5 % 8.1 % 5.2 % 1.7 % — % — % Specialty reinsurance 27.3 % 34.9 % 14.3 % 7.4 % 3.7 % 1.1 % (0.5) % — % — % — %

Income Taxes

Income Taxes	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Income Tax Disclosure [Abstract]
Income Taxes	Twelve Months Ended December 31, 2023 2022 2021 ($ in millions) Income tax (benefit)/expense allocated to net income $ (132.1) $ (78.1) $ 5.3 Income tax expense/(benefit) allocated to other comprehensive income 20.6 (23.9) 0.3 Total income tax (benefit)/expense $ (111.5) $ (102.0) $ 5.6 Income/(loss) from operations before income taxes and income tax expense/(benefit) attributable to that income/(loss) for the twelve months ended December 31, 2023, 2022 and 2021 is provided in the tables below: Twelve Months Ended December 31, 2023 Income Current tax Deferred tax Total income tax ($ in millions) Bermuda (1) $ 148.8 $ — $ (201.1) $ (201.1) U.S. (2) 236.3 52.4 3.0 55.4 U.K. (3) 0.7 5.3 0.1 5.4 Other (4) 16.8 7.9 0.3 8.2 Total $ 402.6 $ 65.6 $ (197.7) $ (132.1) Twelve Months Ended December 31, 2022 (Loss)/income Current tax Deferred tax Total income tax ($ in millions) Bermuda $ (103.3) $ — $ — $ — U.S. 34.8 14.8 (102.9) (88.1) U.K. 62.4 7.0 — 7.0 Other (20.9) 4.7 (1.7) 3.0 Total $ (27.0) $ 26.5 $ (104.6) $ (78.1) Twelve Months Ended December 31, 2021 Income/(loss) Current tax Deferred tax Total income tax ($ in millions) Bermuda $ 22.9 $ — $ — $ — U.S 5.0 5.8 — 5.8 U.K. 75.1 — (0.3) (0.3) Other (67.9) 2.7 (2.9) (0.2) Total $ 35.1 $ 8.5 $ (3.2) $ 5.3 ________________ (1) We have recorded a deferred tax asset in Bermuda consisting of $156.6 million in respect of the ETA and $44.5 million in respect of an OTLC as a result of the newly enacted Corporate Income Tax Act 2023 in Bermuda. The ETA election allows for an adjustment equal to the difference between the fair market value and carrying value of assets and liabilities. The OTLC allows losses from year 2020 to 2024 to be carried forward. We expect this deferred tax asset to be utilized predominantly over a 10-year period. We expect to incur and pay increased taxes in Bermuda beginning in 2025. (2) The U.S. current tax expense of $52.4 million (2022 — $14.8 million) includes $0.9 million of Base Erosion and Anti-abuse Tax. (3) The U.K. current tax expense of $5.3 million largely relates to prior year adjustments. (4) Current tax expense and deferred tax expense in “Other” mostly relates to prior year adjustments in the branches of Aspen UK. As noted above, the tax rate in Bermuda, the Company’s country of domicile, is currently zero. Application of the statutory income tax rate for operations in other jurisdictions produces a differential to the expected income tax (benefit)/expense as shown in the table below. The reconciliation between the income tax (benefit)/expense and the amount that would result from applying the statutory rate for the Company for the twelve months ended December 31, 2023, 2022 and 2021 is provided in the table below: Twelve Months Ended December 31, 2023 2022 2021 Income Tax Reconciliation ($ in millions) Income tax benefit at statutory tax rate of zero percent $ — $ — $ — Overseas statutory tax rates differential 56.3 16.8 (0.9) Base erosion and anti-abuse tax (BEAT) expense 0.9 2.3 6.1 Prior year adjustments (1) 6.9 (2.9) 0.5 Introduction of Bermuda corporate income tax (201.1) — — Change in valuation allowance (2) 4.0 (98.9) 9.6 Impact of unrecognized tax benefits (3) — — — Australian non-resident withholding tax — 1.5 0.6 Foreign exchange (1.3) (0.3) (1.5) Non-deductible expenses 2.5 2.4 2.4 Impact of changes in statutory tax rates (0.3) (5.7) (11.5) Tax effect of OCI in income statement — 6.7 — Total income tax (benefit)/expense $ (132.1) $ (78.1) $ 5.3 ________________ (1) The submission dates for filing income tax returns for the Company’s U.S. and U.K. operating subsidiaries are after the submission date of this report. Accordingly, the final tax liabilities may differ from the estimated income tax expense included in this report and may result in prior year adjustments being reported. The prior period adjustments for the twelve months ended December 31, 2023 predominantly relate to the determination of the results of the branches of the U.K. operating subsidiaries. The prior period adjustments for the twelve months ended December 31, 2022 and 2021 predominantly relate to the determination of results in the U.K. (2) The decrease in valuation allowance in 2022 related to a change in judgment about the recoverability of deferred tax assets in the U.S. operating subsidiaries. (3) In 2023, the Company did not have any unrecognized tax benefits. Income tax returns that have been filed by the Company’s U.S. Operating Subsidiaries are subject to examination for 2020 and later tax years. The Company’s U.K. operating subsidiaries’ income tax returns are potentially subject to examination for 2022 and later tax years as these periods are considered “open” by the U.K. Tax Authority. The Company accrues interest and penalties related to an underpayment of income taxes, if applicable, as income tax expenses. The Company does not believe it will be subject to any penalties in any open tax years. The tax effects of temporary differences and carryforwards that give rise to deferred tax assets and deferred tax liabilities are presented in the following table as at December 31, 2023 and 2022: As at December 31, 2023 2022 ($ in millions) D eferred tax assets: Operating loss carryforwards 217.2 167.6 Capital loss carryforwards 9.7 6.7 Insurance reserves: Losses and loss adjustment expenses 104.1 28.3 Unrealized losses on investments 8.9 20.6 Accrued expenses 13.4 6.1 Foreign tax credit carryforwards 19.0 19.8 Insurance reserves: Unearned premiums 35.0 36.0 Intangible assets 82.9 0.7 Office properties and equipment 16.5 14.2 Operating lease liabilities 15.6 18.5 Other temporary differences 7.6 3.7 Total deferred tax assets 529.9 322.2 Less valuation allowance (172.7) (145.7) Deferred tax assets, net of valuation allowance $ 357.2 $ 176.5 Deferred tax liabilities: Deferred acquisition costs (32.4) (37.0) Right-of-use operating lease assets (10.4) (13.7) Insurance reserves: Losses and loss adjustment expenses (0.1) (0.2) Other temporary differences (3.3) (6.4) Total deferred tax (liabilities) (46.2) (57.3) Net deferred tax assets $ 311.0 $ 119.2	Income Taxes Aspen Holdings and Aspen Bermuda are incorporated under the laws of Bermuda. Under Bermuda law, the corporate tax rate is currently zero and, as a result, Aspen Holdings and Aspen Bermuda are not taxed on any Bermudian income or capital gains. On December 27, 2023, the Corporate Income Tax Act 2023 received Royal Assent in Bermuda, introducing a 15% corporate tax that applies to Bermuda businesses that are part of multinational enterprise groups. This new corporate tax takes effect for accounting periods beginning on or after January 1, 2025. We have adjusted our deferred tax to account for provisions within the Corporate Income Tax Act that allow for an equitable transition to the new regime including the Economic Transition Adjustments (“ETA”) and opening tax loss carryforward (“OTLC”). The Company’s U.S. operating companies were subject to a U.S. federal income tax rate of 21%. The Company’s U.K. operating companies were taxed at the effective U.K. corporate tax rate of 23.5%. The U.K. tax rate changed on April 1, 2023 from 19% to 25%. Total income tax (benefit)/expense for the twelve months ended December 31, 2023, 2022 and 2021 was allocated as follows: Twelve Months Ended December 31, 2023 2022 2021 ($ in millions) Income tax (benefit)/expense allocated to net income $ (132.1) $ (78.1) $ 5.3 Income tax expense/(benefit) allocated to other comprehensive income 20.6 (23.9) 0.3 Total income tax (benefit)/expense $ (111.5) $ (102.0) $ 5.6 Income/(loss) from operations before income taxes and income tax expense/(benefit) attributable to that income/(loss) for the twelve months ended December 31, 2023, 2022 and 2021 is provided in the tables below: Twelve Months Ended December 31, 2023 Income Current tax Deferred tax Total income tax ($ in millions) Bermuda (1) $ 148.8 $ — $ (201.1) $ (201.1) U.S. (2) 236.3 52.4 3.0 55.4 U.K. (3) 0.7 5.3 0.1 5.4 Other (4) 16.8 7.9 0.3 8.2 Total $ 402.6 $ 65.6 $ (197.7) $ (132.1) Twelve Months Ended December 31, 2022 (Loss)/income Current tax Deferred tax Total income tax ($ in millions) Bermuda $ (103.3) $ — $ — $ — U.S. 34.8 14.8 (102.9) (88.1) U.K. 62.4 7.0 — 7.0 Other (20.9) 4.7 (1.7) 3.0 Total $ (27.0) $ 26.5 $ (104.6) $ (78.1) Twelve Months Ended December 31, 2021 Income/(loss) Current tax Deferred tax Total income tax ($ in millions) Bermuda $ 22.9 $ — $ — $ — U.S 5.0 5.8 — 5.8 U.K. 75.1 — (0.3) (0.3) Other (67.9) 2.7 (2.9) (0.2) Total $ 35.1 $ 8.5 $ (3.2) $ 5.3 ________________ (1) We have recorded a deferred tax asset in Bermuda consisting of $156.6 million in respect of the ETA and $44.5 million in respect of an OTLC as a result of the newly enacted Corporate Income Tax Act 2023 in Bermuda. The ETA election allows for an adjustment equal to the difference between the fair market value and carrying value of assets and liabilities. The OTLC allows losses from year 2020 to 2024 to be carried forward. We expect this deferred tax asset to be utilized predominantly over a 10-year period. We expect to incur and pay increased taxes in Bermuda beginning in 2025. (2) The U.S. current tax expense of $52.4 million (2022 — $14.8 million) includes $0.9 million of Base Erosion and Anti-abuse Tax. (3) The U.K. current tax expense of $5.3 million largely relates to prior year adjustments. (4) Current tax expense and deferred tax expense in “Other” mostly relates to prior year adjustments in the branches of Aspen UK. As noted above, the tax rate in Bermuda, the Company’s country of domicile, is currently zero. Application of the statutory income tax rate for operations in other jurisdictions produces a differential to the expected income tax (benefit)/expense as shown in the table below. The reconciliation between the income tax (benefit)/expense and the amount that would result from applying the statutory rate for the Company for the twelve months ended December 31, 2023, 2022 and 2021 is provided in the table below: Twelve Months Ended December 31, 2023 2022 2021 Income Tax Reconciliation ($ in millions) Income tax benefit at statutory tax rate of zero percent $ — $ — $ — Overseas statutory tax rates differential 56.3 16.8 (0.9) Base erosion and anti-abuse tax (BEAT) expense 0.9 2.3 6.1 Prior year adjustments (1) 6.9 (2.9) 0.5 Introduction of Bermuda corporate income tax (201.1) — — Change in valuation allowance (2) 4.0 (98.9) 9.6 Impact of unrecognized tax benefits (3) — — — Australian non-resident withholding tax — 1.5 0.6 Foreign exchange (1.3) (0.3) (1.5) Non-deductible expenses 2.5 2.4 2.4 Impact of changes in statutory tax rates (0.3) (5.7) (11.5) Tax effect of OCI in income statement — 6.7 — Total income tax (benefit)/expense $ (132.1) $ (78.1) $ 5.3 ________________ (1) The submission dates for filing income tax returns for the Company’s U.S. and U.K. operating subsidiaries are after the submission date of this report. Accordingly, the final tax liabilities may differ from the estimated income tax expense included in this report and may result in prior year adjustments being reported. The prior period adjustments for the twelve months ended December 31, 2023 predominantly relate to the determination of the results of the branches of the U.K. operating subsidiaries. The prior period adjustments for the twelve months ended December 31, 2022 and 2021 predominantly relate to the determination of results in the U.K. (2) The decrease in valuation allowance in 2022 related to a change in judgment about the recoverability of deferred tax assets in the U.S. operating subsidiaries. (3) In 2023, the Company did not have any unrecognized tax benefits. Income tax returns that have been filed by the Company’s U.S. Operating Subsidiaries are subject to examination for 2020 and later tax years. The Company’s U.K. operating subsidiaries’ income tax returns are potentially subject to examination for 2022 and later tax years as these periods are considered “open” by the U.K. Tax Authority. The Company accrues interest and penalties related to an underpayment of income taxes, if applicable, as income tax expenses. The Company does not believe it will be subject to any penalties in any open tax years. The tax effects of temporary differences and carryforwards that give rise to deferred tax assets and deferred tax liabilities are presented in the following table as at December 31, 2023 and 2022: As at December 31, 2023 2022 ($ in millions) D eferred tax assets: Operating loss carryforwards 217.2 167.6 Capital loss carryforwards 9.7 6.7 Insurance reserves: Losses and loss adjustment expenses 104.1 28.3 Unrealized losses on investments 8.9 20.6 Accrued expenses 13.4 6.1 Foreign tax credit carryforwards 19.0 19.8 Insurance reserves: Unearned premiums 35.0 36.0 Intangible assets 82.9 0.7 Office properties and equipment 16.5 14.2 Operating lease liabilities 15.6 18.5 Other temporary differences 7.6 3.7 Total deferred tax assets 529.9 322.2 Less valuation allowance (172.7) (145.7) Deferred tax assets, net of valuation allowance $ 357.2 $ 176.5 Deferred tax liabilities: Deferred acquisition costs (32.4) (37.0) Right-of-use operating lease assets (10.4) (13.7) Insurance reserves: Losses and loss adjustment expenses (0.1) (0.2) Other temporary differences (3.3) (6.4) Total deferred tax (liabilities) (46.2) (57.3) Net deferred tax assets $ 311.0 $ 119.2 Deferred tax liabilities and assets represent the tax effect of carryforwards and temporary differences between the value of assets and liabilities for financial statement purposes and such values as measured by U.K., U.S., Bermuda and other tax laws and regulations. In assessing the realizability of deferred tax assets, management considers whether it is more likely than not that some portion or all of the deferred tax assets will not be realized. The ultimate realization of deferred tax assets is dependent upon the generation of future taxable income during the periods in which those temporary differences and carry forwards become deductible or creditable. Management considers the scheduled reversal of existing taxable temporary differences, carryback availability, projected future taxable income, and tax-planning strategies in making this assessment. As at December 31, 2023, the Company has net operating losses carryforwards for U.S. federal income tax purposes of $354.9 million (2022 — $376.5 million), of which $270.2 million relates to the U.S. operating subsidiaries and $84.7 million to Aspen UK’s U.S. branch. The Company also has net operating losses carryforwards for U.K. corporate tax purposes of $248.1 million (2022 — $280.5 million), deferred syndicate profits of $64.5 million (2022 — $19.9 million profits), and losses in other jurisdictions of $97.8 million (2022 — $118.8 million). The $354.9 million that are available to offset future U.S. federal taxable income will expire between 2032 and 2041. The amount of pre-merger net operating losses carryforwards that can be used each year is limited by section 382 to $6.5 million per year for Aspen UK’s U.S. branch, and $39.2 million in 2023, $23.1 million in 2024, and $20.8 million per year for the 15 years thereafter for the U.S. operating subsidiaries. The net operating losses in the U.K. and other jurisdictions are available to offset future corporate income in those jurisdictions over an indefinite period. For U.S. federal income tax purposes, the Company has capital loss carryforwards of $46.1 million, of which $15.8 million relates to the U.S. operating subsidiaries and $30.3 million to Aspen UK’s branch, expiring between 2026 and 2028. For U.K. corporate tax purposes, the Company has foreign tax credit carryforwards of $19.0 million (2022 — $19.8 million) which are available to offset future U.K. corporate tax arising on the same foreign source of income over an indefinite period. A valuation allowance of $24.5 million (2022 — $18.0 million) on U.S. deferred tax assets (which includes these loss carryforwards) has been recognized at December 31, 2023 relating to Aspen UK’s U.S. branch. A valuation allowance of $131.0 million (2022 — $106.7 million) has been established against U.K. deferred tax assets.

Capital Structure

Capital Structure	12 Months Ended
	Dec. 31, 2023
Equity [Abstract]
Capital Structure	Capital Structure The following table provides a summary of the Company’s authorized and issued share capital as at December 31, 2023 and 2022: As at December 31, 2023 At December 31, 2022 Number $ in Number $ in Authorized share capital: Ordinary Shares $0.01 per share (2022 — $0.01 per share) 70,000,000 700 70,000,000 700 Preference Shares 0.15144558¢ per share 30,000,000 45 30,000,000 45 Total authorized share capital 745 745 Issued share capital: Issued ordinary shares $0.01 per share (2022 — $0.01 per share) 60,395,839 604 60,395,839 604 Issued 5.950% preference shares of 0.15144558¢ each with a liquidation preference of $25 per share 11,000,000 17 11,000,000 17 Issued 5.625% preference shares of 0.15144558¢ each with a liquidation preference of $25 per share 10,000,000 15 10,000,000 15 Issued 5.625% preference shares of 0.15144558¢ represented by depositary shares, each with a liquidation preference of $25 per share (1) 10,000 — 10,000 — Total issued share capital 636 636 ________________ (1) Each depositary share represents a 1/1000th interest in a share of the 5.625% preference shares. (a) Ordinary Shares Issued Ordinary Shares. The Company’s issued ordinary shares of par value $0.01 at both December 31, 2023 and 2022 was 60,395,839. The Company did not acquire any ordinary shares for the twelve months ended December 31, 2023. (b) Preference Shares Preference Shares Issuance. On May 2, 2013, the Company issued 11,000,000 5.950% Fixed-to-Floating Rate Perpetual Non-Cumulative Preference Shares, with a liquidation preference of $25 per share (the “AHL PRC Shares”). Net proceeds were $270.6 million, consisting of $275.0 million of total liquidation preference less $4.4 million of issuance expenses. The first floating rate-period commenced July 1, 2023, with an associated floating rate of 9.59343% with such floating rate expected to remain in place for future dividend periods, as a function of the floating rate determination mechanics set forth in the governing instrument. The AHL PRC Shares are listed on the NYSE under symbol “AHLPRC”. On September 20, 2016, the Company issued 10,000,000 shares of 5.625% Perpetual Non-Cumulative Preference Shares (the “AHL PRD Shares”). The 2016 Preference Shares have a liquidation preference of $25 per share. Net proceeds were $241.3 million, consisting of $250.0 million of total liquidation preference less $8.7 million of issuance expenses. The AHL PRD Shares are listed on the NYSE under the symbol “AHL PRD”. On August 13, 2019, the Company issued 10,000,000 depositary shares, each of which represents 1/1000th interest in a share of the newly designated 5.625% Perpetual Non-Cumulative Preference Shares. The depositary shares have a liquidation preference of $25 per share. Net proceeds were $241.6 million, comprising $250.0 million of total liquidation preference less $8.4 million of issuance expenses. The depositary shares are listed on the NYSE under the symbol “AHL PRE”.

Earnings Per Share

Earnings Per Share	12 Months Ended
	Dec. 31, 2023
Earnings Per Share [Abstract]
Earnings Per Ordinary Share	Earnings Per Ordinary Share In December 2023, the Company filed a registration statement with the SEC relating to a proposed Initial Public Offering of our ordinary shares. As a result of that filing, and in accordance with the accounting guidance of ASC Topic 260, “Earnings Per Share” , the Company has presented the following disclosure on earnings per ordinary share. This disclosure is new for the fiscal year ended December 31, 2023. Basic and diluted earnings per ordinary share are calculated by dividing net income available to holders of Aspen Insurance Holdings Limited’s ordinary shares by the weighted average number of ordinary shares outstanding. The following table presents the computation of basic and diluted earnings/(loss) per ordinary share for the twelve months ended December 31, 2023, 2022 and 2021. Twelve Months Ended December 31, 2023 2022 2021 ($ in millions, except share and share amounts) Net income $ 534.7 $ 51.1 $ 29.8 Less: Preference share dividends (49.9) (44.6) (44.5) Net income/(loss) available to ordinary shareholders $ 484.8 $ 6.5 $ (14.7) Basic and diluted weighted average ordinary shares outstanding 60,395,839 60,395,839 60,395,839 Basic and diluted earnings/(loss) per ordinary share $ 8.03 $ 0.11 $ (0.24)

Statutory Requirements and Divi

Statutory Requirements and Dividends Restrictions	12 Months Ended
	Dec. 31, 2023
Equity [Abstract]
Summary of Statutory Requirements and Dividends Restrictions	Statutory Requirements and Dividends Restrictions As a holding company, the Company relies on dividends and other distributions from its Operating Subsidiaries to provide cash flow to meet ongoing cash requirements, including any future debt service payments and other expenses, and to pay dividends, if any, to our preference and ordinary shareholders. The Company must comply with the provisions of the Bermuda Companies Act 1981, as amended, (the “Companies Act”) regulating the payment of dividends and distributions. The ability of the Company’s Operating Subsidiaries to pay the Company dividends or other distributions is subject to the laws and regulations applicable to each jurisdiction, as well as the Operating Subsidiaries’ need to maintain capital requirements adequate to maintain their insurance and reinsurance operations and their financial strength ratings issued by independent rating agencies. The company law of England and Wales prohibits Aspen UK, AMAL or AUL from declaring a dividend to its shareholders unless it has “profits available for distribution”. The determination of whether a company has profits available for distribution is based on its accumulated realized profits and other distributable reserves less its accumulated realized losses. While the U.K. insurance regulatory laws impose no statutory restrictions on a general insurer’s ability to declare a dividend, the rules of the Prudential Regulation Authority (the “PRA”) require each insurance company within its jurisdiction to maintain its solvency margin at all times. Accordingly, Aspen UK, AMAL and AUL may not pay a dividend if the payment of such dividend would result in their SCR coverage ratio falling below certain levels. In addition, any future changes regarding regulatory requirements, including those described above, may restrict the ability of Aspen UK, AMAL and AUL to pay dividends in the future. As at December 31, 2023, Aspen UK had an accumulated balance of retained losses of approximately $599.3 million and AUL had an accumulated balance of retained losses of approximately £78 million. Aspen UK held a capital contribution reserve of $655.0 million as at December 31, 2023 which, under certain circumstances, could be distributable. Aspen Bermuda must comply with the provisions of the Companies Act and the Insurance Act regulating the payment of dividends and distributions. Aspen Bermuda may not in any financial year pay dividends which would exceed 25% of its total statutory capital and surplus, as shown on its statutory balance sheet in relation to the previous financial year, unless it files with the BMA a solvency affidavit at least seven days in advance of payment. As at December 31, 2023, 25% of Aspen Bermuda’s statutory capital and surplus amounted to $303.9 million. Aspen Bermuda must also obtain the prior approval of the BMA before reducing its total statutory capital as set out in its previous year’s financial statements by 15% or more. Under both North Dakota and Texas law, insurance companies may only pay dividends out of earned surplus as distinguished from contributed surplus. As such, Aspen Specialty and AAIC could not pay a dividend as at December 31, 2023 without prior regulatory approval. Actual and required statutory capital and surplus for the principal Operating Subsidiaries of the Company, excluding its Lloyd’s syndicate, as at December 31, 2023 and December 31, 2022 were estimated as follows: As at December 31, 2023 U.S. Bermuda U.K. ($ in millions) Required statutory capital and surplus $ 488.9 $ 601.1 $ 257.2 Actual statutory capital and surplus $ 1,063.1 $ 1,685.2 $ 734.7 As at December 31, 2022 U.S. Bermuda U.K. ($ in millions) Required statutory capital and surplus $ 513.9 $ 536.7 $ 771.8 Actual statutory capital and surplus $ 838.6 $ 1,426.6 $ 802.5 As the sole corporate member of our Lloyd’s Syndicate, AUL was required to hold capital at Lloyd’s of $989.9 million as at December 31, 2023, adjusting funding to meet this level on an annual basis in the following Q2 and not holding less than 90% of this amount at any time. As at December 31, 2023, AUL had capital at Lloyd’s of $1,101.0 million of which $515.4 million was provided as Funds at Lloyd’s by Aspen Bermuda. The Bermuda Monetary Authority is the group supervisor of the Company. The laws and regulations of Bermuda require that the Company maintain a minimum amount of group statutory capital and surplus based on the enhanced capital requirement using the group standardized risk-based capital model of the Bermuda Monetary Authority. The Company is also subject to an early-warning level based on 120% of the enhanced capital requirement which may trigger additional reporting requirements or other enhanced oversight. The statutory capital requirements of the Company’s Operating Subsidiaries are set out above. To the extent that these requirements are met, the Company do not anticipate any dividend restrictions arising as a result of the Company’s enhanced capital requirement.

Dividends

Dividends	12 Months Ended
	Dec. 31, 2023
Dividends [Abstract]
Preferred Stock [Text Block]	Dividends Dividends. In the twelve months ended December 31, 2023, the Company’s Board of Directors paid the following dividends: Calendar Quarter Preference Share Category Quarterly Total Declared Paid 5.950% PS 5.625% PS 5.625% DS Q1 2023 $ 4,090,900 $ 3,516,000 $ 3,515,600 $ 11,122,500 03/01/23 04/01/23 Q2 2023 $ 4,090,900 $ 3,516,000 $ 3,515,600 $ 11,122,500 06/01/23 07/01/23 Q3 2023 $ 6,815,600 $ 3,516,000 $ 3,515,600 $ 13,847,200 09/01/23 10/01/23 Q4 2023 $ 6,741,900 $ 3,516,000 $ 3,515,600 $ 13,773,500 11/30/23 12/28/23 Total Paid $ 21,739,300 $ 14,064,000 $ 14,062,400 $ 49,865,700 ______________ (1) 5.950% Preference Shares (AHL PRC) — Fixed to Floating Rate Perpetual Non-Cumulative Preference Shares 5.625% Preference Shares (AHL PRD) — Perpetual Non-Cumulative Preference Shares 5.625% Preference Shares(AHL PRE) are represented by depositary shares, each representing a 1/1000 th interest in a share of the 5.625% Preference Shares. The dividend paid per depositary share is likewise 1/1000 th of the declared dividend, equivalent to $0.35156 per depositary share. In the twelve months ended December 31, 2023, the Company paid an ordinary shares dividend of $40.3 million to Highlands Bermuda Holdco, Ltd., the holder of all the Company’s ordinary shares.

Retirement Plans

Retirement Plans	12 Months Ended
	Dec. 31, 2023
Retirement Benefits [Abstract]
Retirement Plans	Retirement Plans

Share-Based Payments and Long-T

Share-Based Payments and Long-Term Incentive Plan	12 Months Ended
	Dec. 31, 2023
Share-Based Payment Arrangement [Abstract]
Share Based Payments and Long-term Incentive Plan	Share-Based Payments and Long-Term Incentive Plan In 2019, the Company implemented a new long-term incentive scheme, under which annual awards are split equally between Performance Units and Exit Units. Performance units vesting conditions have been amended in the current year and vest after two years subject to the Company achieving certain thresholds of operating income over a two year period. Exit Units vest upon change of control (sale or IPO) and achieving predetermined multiplies of invested capital return targets. Both Performance Units and Exit Units are cash-based awards. The Company’s total share-based compensation/long-term incentive plan expense for the twelve months ended December 31, 2023 was $5.5 million (December 31, 2022 — $0.6 million), which is related to a charge of $5.5 million (December 31, 2022 — $0.6 million) in relation to Performance Units. The income tax effect of this is not considered to be material. As at December 31, 2023, the Company had recorded a payable of $7.6 million (December 31, 2022 — $1.6 million) related to the long-term incentive plan, which is included within accrued expenses and other payables in the consolidated balance sheet. Management Equity Plan During 2023, selected senior employees were granted Management Equity Plan (“MEP”) stock options to acquire non-voting shares at a management equity vehicle affiliated with the Company at no cost to the employee. The stock options vest at the later of (a) certification of the attainment of the underlying operating income goal and (b) the exit or liquidity event, with vesting subject to an exit or liquidity event occurring, a two-year cumulative operating income hurdle being achieved over the cumulative two years ending December 31, 2024, and certain other contractual terms being achieved. The weighted average exercise price of the options is $0.001 and the total number of options granted was 10,000. All of the options were granted in 2023, none vested in 2023, none were forfeited and all remain outstanding as of December 31, 2023.

Intangible Assets and Goodwill

Intangible Assets and Goodwill	12 Months Ended
	Dec. 31, 2023
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible Assets and Goodwill	Intangible Assets and Goodwill Aspen’s intangible assets relate to trademarks and licenses to trade in the U.S. and U.K. For the twelve months ended December 31, 2023 and December 31, 2022, the Company had intangible assets and goodwill totalling $21.7 million and $21.8 million. The “Aspen” trademark, valued at $1.1 million, $16.7 million of insurance licenses and $3.9 million of goodwill are considered to have an indefinite life and are tested annually for impairment or when events or changes in circumstances indicate that these assets might be impaired. For the years ended December 31, 2023 and December 31, 2022, the Company performed its annual qualitative assessment and determined that it was more likely than not that these were not impaired.

Operating Leases (Notes)

Operating Leases (Notes)	12 Months Ended
	Dec. 31, 2023
Leases [Abstract]
Lessee, Operating Leases	Operating Leases As at December 31, 2023, the Company has recognized right-of-use operating lease assets of $61.6 million, net of impairment and operating lease liabilities of $86.1 million. Right-of-use operating lease assets comprise primarily of leased office real estate globally and other assets. For all office real estate leases, rent incentives, including reduced-rent and rent-free periods and contractually agreed rent increases during the lease term, have been included when determining the present value of future cash flows. As part of the Company’s operating effectiveness and efficiency program, the Company has consolidated its office space. Where negotiations are either in advanced stages of discussion and it is probable that the sub-lease transactions will be completed, or the Company has agreed terms to sub-lease our office space, the Company has assessed the right-of-use lease assets for impairment. During the twelve months ended December 31, 2023, no impairment has been recognized on the right-of-use lease asset (2022 — $6.7 million credit). The Company has no lease transactions between related parties. Operating lease charge. The following table summarizes the operating lease charge for the twelve months ended December 31, 2023, 2022 and 2021: For the Twelve Months Ended December 31, 2023 December 31, 2022 December 31, 2021 ($ in millions) Amortization charge on right-of-use operating leased assets $ 10.7 $ 10.1 $ 12.0 Interest on operating lease liabilities 4.5 5.4 5.5 Operating lease charge $ 15.2 $ 15.5 $ 17.5 Lease Liabilities. The following table summarizes the maturity of lease liabilities under non-cancellable leases as of December 31, 2023 and 2022: December 31, 2023 December 31, 2022 ($ in millions) Operating leases — maturities 2023 $ — $ 15.4 2024 15.4 15.2 2025 15.1 14.9 2026 14.3 14.1 2027 12.8 12.6 2028 12.7 12.4 Later years 32.4 34.8 Total minimum lease payments $ 102.7 $ 119.4 Less imputed interest (16.6) (23.9) Total lease liabilities $ 86.1 $ 95.5 Other lease information. The following table summarizes the cash flows on operating leases for the twelve months ended December 31, 2023, 2022 and 2021 and other supplemental information: For the Twelve Months Ended December 31, 2023 December 31, 2022 December 31, 2021 ($ in millions) Cash paid for amounts included in the measurement of lease liabilities - Operating cash outflow from operating leases $ (15.5) $ (15.5) $ (17.5) Right-of-use assets obtained in exchange for lease obligations - Operating leases $ 0.2 $ 1.9 $ 23.9 Reduction to Right-of-use assets resulting from reductions to lease obligations - Operating leases $ 0.1 $ 7.0 $ 2.3 Weighted Averages - Operating leases, remaining lease terms (years) 7.3 8.1 8.8 - Operating leases, average discount rate 5.0 % 5.0 % 5 %

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2023
Related Party Transactions [Abstract]
Related Party Transactions	Related Party Transactions Apollo’s indirect subsidiary, Apollo Asset Management Europe PC LLP (“AAME”), serves as the investment manager for the Company and certain of the Company’s subsidiaries, and Apollo’s indirect subsidiary, Apollo Management Holdings, L.P. (“AMH”), provides the Company with management consulting services and advisory services. Additionally, certain employees of Apollo and its affiliates serve on the Board. A description of relationships and transactions that have existed or that the Company and certain of the Company’s subsidiaries has entered into with Apollo and its affiliates are described below. Investment Management Relationships AAME provides centralized asset management investment advisory and risk services for the portfolio of the Company’s investments and investments of such subsidiaries pursuant to the investment management agreements (“IMAs”) that have been entered into with AAME. In addition, pursuant to the IMAs, AAME may engage sub-advisors or delegates to provide certain of the investment advisory and management services to the Company’s subsidiaries. Such sub-advisors may include affiliates of AAME. Under each of the IMAs, AAME will be paid an annual investment management fee (the “Management Fee”) which will be based on a cost-plus structure. The “cost” is comprised of the direct and indirect fees, costs, expenses and other liabilities arising in or otherwise connected with the services provided under the IMAs. The “plus” component will be a mark-up in an amount of up to 25% determined based on an applicable transfer pricing study. The Management Fee will be subject to certain maximum threshold levels, including an annual fee cap of 15 bps of the total amount of investable assets. Affiliated sub-advisors, including AMI and AMC, will also earn additional fees for sub-advisory services rendered. During the year ended December 31, 2023, the Company recognized IMA fees of $9.4 million (2022 — $4.9 million; 2021 — $5.8 million), of which $2.1 million (2022 — $4.5 million) remains payable to AAME at year end. Management Consulting Agreement As previously disclosed, the Company entered into a Management Consulting Agreement, dated March 28, 2019 (the “Management Consulting Agreement”), with AMH. Pursuant to the Management Consulting Agreement, AMH will provide the Company management consulting and advisory services related to the business and affairs of the Company and its subsidiaries. The Company will pay AMH in consideration for its services under the Management Consulting Agreement, an annual management consulting fee equal to the greater of (i) 1% of the consolidated net income of the Company and its subsidiaries for the applicable fiscal year, or (ii) $5 million. During the year ended December 31, 2023, the Company recognized Management Consulting fees of $5.0 million (2022 — $5.0 million; 2021 — $5.0 million), of which $1.2 million remains payable to AMH at year end (2022 — $1.3 million). Related Party Investments During the year, the Company bought or held the following securities or investments in Apollo: As at December 31, 2023, the Company’s investment in Funds managed by Apollo had a fair value of $39.8 million (2022 — $38.0 million). These investments are included in other investments on the consolidated balance sheet. The Company incurred losses of $0.4 million (2022 — gains of $3.1 million) and they are included in net investment income on the consolidated statement of operations and other comprehensive income. There were no expenses incurred relating to any of these investments. Expenses as outlined in the previous sentence and subsequent disclosures in this Related Party Investments section relate to investment management fees paid to Apollo. As at December 31, 2023, the Company’s investment in Notes issued by special purpose vehicles (SPVs) established and managed by subsidiaries of Apollo had a fair value of $82.2 million (2022 — $44.8 million). The Company recognized income of $5.5 million (2022 — losses of $0.4 million) which is included in the consolidated statement of operations and other comprehensive income. These investments are included in privately-held investments on the consolidated balance sheet. There were no expenses incurred relating to any of these investments. As at December 31, 2023, the Company’s investments in Collateralized Loan Obligations (“CLOs”) issued by special purpose vehicles established and managed by subsidiaries of Apollo had a fair value of $129.8 million (2022 — $Nil). Income earned on these investments was $17.4 million (2022 — $Nil) and is included in the consolidated statement of operations and other comprehensive income. These investments are included in fixed income maturities, trading at fair value on the consolidated balance sheet. For the year ended December 31, 2023, the Company incurred expenses of $0.5 million related to these investments. As at December 31, 2023, the Company’s investments in Middle Market Loans originated and managed by a subsidiary of Apollo had a fair value of $45.1 million (2022 — $Nil). The Company recognized income of $5.8 million (2022 — $Nil) which is included in the consolidated statement of operations and other comprehensive income. The Middle Market Loans are included in privately-held investments on the consolidated balance sheet. For the year ended December 31, 2023, the Company incurred expenses of $0.2 million related to these investments. The above transactions were entered into at arm’s length. Other Payables to Related Parties

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2023
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingent Liabilities (a) Restricted assets The Company’s subsidiaries are obliged by the terms of its contractual obligations to U.S. policyholders and by obligations to certain regulatory authorities to facilitate issue of letters of credit or maintain certain balances in trust funds for the benefit of policyholders. The following table details the forms and value of Company’s material restricted assets as at December 31, 2023 and 2022: As at December 31, 2023 At December 31, 2022 ($ in millions, except percentages) Regulatory trusts and deposits: Affiliated transactions $ 660.8 $ 707.1 Third party 2,714.4 2,817.7 Letters of credit / guarantees (1) 172.0 471.3 Total restricted assets (excluding illiquid assets) 3,547.2 3,996.1 Other investments — illiquid assets 209.3 221.3 Total restricted assets and illiquid assets $ 3,756.5 $ 4,217.4 Total as percent of cash and invested assets (2) 50.2 % 59.4 % ________________ (1) As at December 31, 2023, the Company had pledged funds of $172.0 million (December 31, 2022 — $471.3 million) as collateral for the secured letters of credit. (2) Investable assets comprise total investments, cash and cash equivalents, accrued interest, receivables for securities sold and payables for securities purchased. Investment Funds. We invest in investment funds which, as is typical for this type of investment, have lock-up periods. A lock-up period is the initial amount of time an investor is contractually required to remain invested before having the ability to redeem. As at December 31, 2023, the lock-up periods across these funds range from one quarter to several years. Thereafter these funds could also be redeemed on a pro-rata basis depending on the liquidity position of the fund. There are no assurances as to when the Company may be able to withdraw, in whole or in part, its redemption request from the fund. Other Investments - Equity Method. On December 23, 2019, the Company committed $5.0 million as an equity investment in the holding company of a multi-line reinsurer. The strategy for the multi-line reinsurer is to combine a diversified reinsurance business, focused primarily on long-tailed lines of property and casualty business and, potentially to a lesser extent, life business, with a diversified investment strategy. During the period ending December 31, 2023, $0.4 million (December 31, 2022 — $1.6 million) capital was invested in multi-line reinsurer. The Company’s current arrangements with our bankers for the issue of letters of credit require us to provide collateral in the form of cash and investments for the full amount of all secured and undrawn letters of credit that are outstanding. We monitor the proportion of our otherwise liquid assets that are committed to trust funds or to the collateralization of letters of credit. As at December 31, 2023 and 2022, these funds amounted to approximately 50.2% of the $7.5 billion and approximately 59.4% of the $7.1 billion of investable assets held by the Company, respectively. We do not consider that this unduly restricts our liquidity at this time. For more information on our credit facilities and long-term debt arrangements, refer to Note 24, “Credit Facility and Long-term Debt” of these consolidated financial statements. Funds at Lloyd’s. AUL operates at Lloyd’s as the corporate member for Syndicate 4711. AUL also participates in underwriting activities of Carbon Syndicate 4747. Lloyd’s determines required regulatory capital by considering the underwriting activities that AUL participates in. Such capital, called Funds at Lloyd’s, consists of investable assets as at December 31, 2023 in the amount of $517.4 million (2022 — $489.5 million). The amounts provided as Funds at Lloyd’s will be drawn upon and become a liability of the Company in the event of Syndicate 4711 declaring a loss at a level that cannot be funded from other resources, or if Syndicate 4711 requires funds to cover a short-term liquidity gap. The amount which the Company provides as Funds at Lloyd’s is not available for distribution to the Company for the payment of dividends. Aspen Managing Agency Limited, the managing agent to Syndicate 4711, is also required by Lloyd’s to maintain a minimum level of capital which as at December 31, 2023 was £0.5 million (December 31, 2022 — £0.4 million). This is not available for distribution by the Company for the payment of dividends. U.S. Reinsurance Trust Fund. For its U.S. reinsurance activities, Aspen UK has established and must retain a multi-beneficiary U.S. trust fund for the benefit of its U.S. cedants so that they may take financial statement credit without the need to post cedant-specific security. The minimum trust fund amount is $20.0 million plus an amount equal to 100% of Aspen UK’s U.S. reinsurance liabilities, which were $823.5 million as at December 31, 2023 and $1,166.4 million as at December 31, 2022. As at December 31, 2023, the balance (including applicable letter of credit facilities) held in the trust was $1,016.9 million (2022 — $1,389.5 million). Aspen Bermuda has also established and must retain a multi-beneficiary U.S. trust fund for the benefit of its U.S. cedants so that they may take financial statement credit without the need to post cedant-specific security. The minimum trust fund amount is $20.0 million plus an amount equal to 100% of Aspen Bermuda’s liabilities to its U.S. cedants which was $320.6 million and $380.3 million as at December 31, 2023 and 2022, respectively. As at December 31, 2023, the balance held in the U.S. trust fund and other Aspen Bermuda trusts was $394.7 million (2022 — $509.2 million). U.S. Surplus Lines Trust Fund. Aspen UK and Syndicate 4711 have also established a U.S. surplus lines trust fund with a U.S. bank to secure liabilities under U.S. surplus lines policies. The balance held in trust as at December 31, 2023 was $126.6 million (2022 — $215.1 million). U.S. Regulatory Deposits. As at December 31, 2023, Aspen Specialty had a total of $6.8 million (2022 — $6.7 million) on deposit with six U.S. states in order to satisfy state regulations for writing business in those states. AAIC had a further $6.4 million (2022 — $6.5 million) on deposit with twelve U.S. states. Canadian Trust Fund. Aspen UK has established a Canadian trust fund with a Canadian bank to secure a Canadian insurance license. As at December 31, 2023, the balance held in trust was CAD$228.4 million ($168.4 million) (2022 — CAD$185.8 million). Australian Trust Fund. Aspen UK has established an Australian trust fund with an Australian bank to secure policyholder liabilities and as a condition for maintaining an Australian insurance license. As at December 31, 2023, the balance held in trust was AUD$131.0 million ($86.9 million) (2022 — AUD$183.9 million). Swiss Trust Fund. Aspen UK has established a Swiss trust fund with a Swiss bank to secure policyholder liabilities and as a condition for maintaining a Swiss insurance license. As at December 31, 2023, the balance held in trust was CHF9.9 million ($11.4 million) (2022 — CHF9.4 million). Singapore Fund. Aspen UK has established a segregated Singaporean bank account to secure policyholder liabilities and as a condition for maintaining a Singaporean insurance license and meet local solvency requirements. As at December 31, 2023, the balance in the account was SGD$192.1 million ($144.0 million) (2022 — SGD$174.7 million). (b) Contingent liabilities In common with the rest of the insurance and reinsurance industry, the Company is also subject to litigation and arbitration in the ordinary course of business. The Company’s Operating Subsidiaries are regularly engaged in the investigation, conduct and defense of disputes, or potential disputes, resulting from questions of insurance or reinsurance coverage or claims activities. Pursuant to insurance and reinsurance arrangements, many of these disputes are resolved by arbitration or other forms of alternative dispute resolution. Such legal proceedings are considered in connection with estimating the Company’s Insurance Reserves — Loss and Loss Adjustment Expenses, as provided on the Company’s consolidated balance sheet. In some jurisdictions, noticeably the U.S., a failure to deal with such disputes or potential disputes in an appropriate manner could result in an award of “bad faith” punitive damages against the Company’s Operating Subsidiaries. In accordance with ASC 450-20-50-3, for (a) reasonably possible losses for which no accrual is made because any of the conditions for accrual in ASC 450-20-25-2 are not met and (b) reasonably possible losses in excess of the amounts accrued pursuant to ASC 450-20-30-1, the Company will provide an estimate of the possible loss or range of possible loss or state that such an estimate cannot be made. As at December 31, 2023, based on available information the probability of the ultimate resolution of pending or threatened litigation or arbitrations having a material effect on the Company’s financial condition, results of operations or liquidity is remote.

Concentration of Credit Risk

Concentration of Credit Risk	12 Months Ended
	Dec. 31, 2023
Risks and Uncertainties [Abstract]
Concentrations of Credit Risk	Concentrations of Credit Risk The Company is potentially exposed to concentrations of credit risk in respect of amounts recoverable from reinsurers, investments and cash and cash equivalents, and insurance and reinsurance balances owed by the brokers with whom the Company transacts business. The Company defines credit risk tolerances in line with the risk appetite set by our Board and they, together with the group’s risk management function, monitor exposures to individual counterparties. Any exceptions are reported to senior management and the Risk Committee of the Board of Directors. Reinsurance recoverables At December 31, 2023, the total amount recoverable by the Company from reinsurers was $4,577.8 million (December 31, 2022 — $4,897.7 million). Of the Company’s reinsurance recoverable balance at December 31, 2023, 56.8% is collateralized by our reinsurers, 42.9% is recoverable from reinsurers rated A- or higher by major rating agencies and 0.3% is recoverable from reinsurers rated lower than A- by major rating agencies (December 31, 2022 — 57.3%, 42.3% and 0.4%, respectively). As at December 31, 2023, the Company’s largest uncollateralized exposures to individual reinsurers represent 15.9% (December 31, 2022 —16.3%), 11.1% (December 31, 2022 — 9.7%), and 9.2% (December 31, 2022 — 8.2%). Under the current expected credit loss model (“CECL”), the Company recognized a provision against reinsurance recoverables of $3.7 million as at December 31, 2023 (December 31, 2022 — $3.7 million). For the twelve months ended December 31, 2023, there was a no change in the CECL allowance on reinsurance recoverables. Underwriting premium receivables The total underwriting premium receivable by the Company as at December 31, 2023 was $1,435.3 million (2022 — $1,482.4 million). As at December 31, 2023, $8.7 million of the total underwriting premium receivable balance has been due for settlement for more than one year. The Company assesses the recoverability of premium receivables through a review of policies and the concentration of receivables by broker. The Company has recognized an allowance for credit losses of $21.0 million as at December 31, 2023 (December 31, 2022 — $25.0 million) on underwriting premium receivables. Investments and cash and cash equivalents The Company’s investment policies include specific provisions that limit the allowable holdings of a single issue and issuer. As at December 31, 2023, there were no investments in any single issuer, other than the U.S. government and the Canadian government in excess of 2% of the aggregate investment portfolio.

Reclassifications from Accumula

Reclassifications from Accumulated Other Comprehensive Income	12 Months Ended
	Dec. 31, 2023
Equity [Abstract]
Reclassifications from Accumulated Other Comprehensive Income	Reclassifications from Accumulated Other Comprehensive Income The following table sets out the components of the Company’s AOCI that are reclassified into the condensed consolidated statement of operations for the twelve months ended December 31, 2023, 2022 and 2021: Amount Reclassified from AOCI Details about the AOCI Components Twelve Months Ended December 31, 2023 Twelve Months Ended December 31, 2022 Twelve Months Ended December 31, 2021 Affected Line Item in the ($ in millions) Available for sale: Realized (gains) on sale of securities $ (2.2) $ (3.9) $ (24.8) Realized and unrealized investment gains Realized losses on sale of securities 42.4 58.9 4.4 Realized and unrealized investment losses 40.2 55.0 (20.4) Income from operations before income tax Tax on net realized gains of securities (6.6) — — Income tax (expense)/benefit $ 33.6 $ 55.0 $ (20.4) Net income Realized derivatives: Net realized gains on settled derivatives (8.1) 15.4 (6.2) General, administrative and corporate expenses Tax on settled derivatives — — — Income tax (expense)/benefit $ (8.1) $ 15.4 $ (6.2) Net income Total reclassifications from AOCI to the statement of operations, net of income tax $ 25.5 $ 70.4 $ (26.6) Net income

Credit Facilities and Long-term

Credit Facilities and Long-term Debt	12 Months Ended
	Dec. 31, 2023
Debt Disclosure [Abstract]
Credit Facilities and Short-term Debt	Credit Facilities and Long-term Debt In the normal course of its operations, the Company enters into agreements with financial institutions to obtain financing through secured and unsecured credit facilities. As at December 31, 2023, the total capital under such facilities available to the Company was approximately $2.3 billion, with the significant facilities as follows: Credit Facilities (i) On December 1, 2021, Aspen Holdings and certain of its direct or indirect subsidiaries (collectively, the “Borrowers”) entered into a Third Amended and Restated Credit Agreement, as further amended from time to time (the “Credit Agreement”) with various lenders and Barclays Bank plc, as administrative agent, which amends and restates the Amended and Restated Credit Agreement, dated as of June 12, 2013 and the Second Amended and Restated Credit Agreement, dated as of March 27, 2017, among Aspen Holdings, certain subsidiaries thereof, various lenders and Barclays Bank plc, as administrative agent. The Credit Agreement will be used by the Borrowers to finance the working capital needs of the Aspen Holdings and its subsidiaries, for letters of credit in connection with the insurance and reinsurance businesses of the Company and its subsidiaries and borrowings for other general corporate purposes. Initial availability under the Credit Agreement is $300,000,000 and the Company has the right to request (subject to the terms and conditions of the Credit Agreement) an increase to the credit facility by up to $100,000,000. The Credit Agreement will expire on December 1, 2026. As at December 31, 2023, there were no borrowings outstanding under the Credit Agreement. The fees and interest rates on the loans and the fees on the letters of credit payable by the Borrowers under the Credit Agreement are based upon the credit ratings for the Company’s long-term unsecured senior, non-credit enhanced debt rating of the Company, as determined by S&P and Moody’s. In addition, the fees for a letter of credit vary based upon whether the applicable Borrower has provided collateral (in the form of cash or qualifying debt securities) to secure its reimbursement obligations with respect to such letter of credit. Under the Credit Agreement, the Company must not permit (a) consolidated tangible net worth as at the last day of each fiscal quarter of the Company to be less than the sum of (i) $2,019,600,000, (ii) 25% of consolidated net income during the period from January 1, 2021 to and including such last day of such fiscal quarter (if positive) and (iii) 25% of the aggregate net cash proceeds of all issuances by the Company of shares of its capital stock during the period from January 1, 2021 to and including such last day of such fiscal quarter, but excluding (x) any amount included in the Company’s accumulated other comprehensive income or loss related to unrealized gains or losses on available for sale securities and (y) during the period from January 1, 2022, any amount included in net unrealized investment gains or losses, related to unrealized gains or losses on trading securities, (b) the ratio of its total consolidated debt to the sum of such debt plus our consolidated tangible net worth to exceed 35% as at the last day of any fiscal quarter of the Company or (c) any material insurance subsidiary to have a financial strength rating of less than “B++” from A.M. Best. The Credit Agreement contains other customary affirmative and negative covenants, including (subject to various exceptions) restrictions on the ability of the Company and its subsidiaries to incur indebtedness, create or permit liens on their assets, engage in mergers or consolidations, dispose of assets, pay dividends or other distributions, purchase or redeem the Company’s equity securities, make investments and enter into transactions with affiliates. In addition, the Credit Agreement has customary events of default, including (subject to certain materiality thresholds and grace periods) payment default, failure to comply with covenants, material inaccuracy of representation or warranty, bankruptcy or insolvency proceedings, change of control and cross-default to other debt agreements. Other Credit Facilities. (ii) On February 7, 2019, Aspen European and Aspen Holdings (acting as guarantor of Aspen European) entered into a letter of credit facility for the purpose of obtaining a letter of credit in favor of Aspen UK for a sum not to exceed $100 million to provide approved regulatory capital for Aspen UK. A letter of credit was issued in favor of Aspen UK for a sum of $100 million. This facility was amended and restated with respect to February 7, 2023, pursuant to which the $100 million letter of credit was extended to February 11, 2027. (iii) On October 24, 2023, AUL and Aspen Holdings (acting as guarantor of AUL), effected an amendment to a letter of credit facility agreement for the account of AUL, pursuant to which a syndicate of lenders issued a several letter of credit in an aggregate amount of $335,000,000, for the benefit of Lloyd’s, to support AUL’s Funds at Lloyd’s requirements in connection with the 2024 year of account at Lloyd’s. This further amended the letter of credit facility agreement, dated November 3, 2020, entered into between AUL, Aspen Holdings (acting as guarantor of AUL) and various lenders, for the account of AUL, pursuant to which a lender provided a maximum aggregate amount of $235,000,000, to support AUL’s Funds at Lloyd’s requirements in connection with the 2021 year of account at Lloyd’s, as amended on May 7, 2021, November 1, 2021, May 6, 2022 and October 27, 2022, in connection with the 2021, 2022 and 2023 underwriting years of account at Lloyd’s, as applicable. (iv) On November 29, 2023, AUL and Aspen Holdings (acting as guarantor of AUL) amended a Funds at Lloyd’s Facility Agreement dated November 25, 2020, as amended on December 2, 2021 and as further amended on December 1, 2022, for the account of AUL. This facility provides that a maximum aggregate amount of up to $80.0 million of acceptable securities may be deposited with, and for the benefit of, Lloyd’s on behalf of AUL to support AUL’s Funds at Lloyd’s requirements in connection with the 2024 year of account at Lloyd’s. (v) On November 30, 2023, AUL and Aspen Bermuda (acting as AUL’s guarantor) amended and restated a Funds a Lloyd’s Facility Agreement originally dated November 30, 2020, as amended on November 30, 2021 and as amended and restated on December 2, 2022, for the account of AUL. This facility provides that a maximum aggregate amount of up to $150 million of acceptable securities may be deposited with, and for the benefit of, Lloyd’s on behalf of AUL to support AUL’s Funds at Lloyd’s requirements in connection with the 2024 year of account at Lloyd’s. (vi) On April 1, 2021, the Company’s subsidiaries, AAIC and Aspen Specialty, each established a secured line of credit at Federal Home Loan Bank of Boston (“FHLBB”). Advances may be used to support general corporate purposes. The maximum amount available under these facilities will vary based on the borrower’s net admitted assets or reserve assets (total invested assets) and the lender’s underwriting criteria. Aspen Specialty’s maximum borrowing capacity available from FHLBB upon initial application is 15% of net admitted assets or approximately $250 million, and is subject to North Dakota approval. Under Texas state insurance law, without the prior consent of the Texas Department of Insurance, the amount of assets AAIC may pledge to secure debt obligations is limited to 10% of its reserve assets, resulting in a maximum borrowing capacity for AAIC under its FHLBB facility of approximately $174 million. Neither AAIC nor Aspen Specialty expects to draw on these facilities in the near future. (vii) On November 5, 2021, Aspen Holdings entered into a letter of credit facility agreement. The letter of credit issued under this facility is the for the benefit of Aspen Bermuda, as beneficiary, and has been applied towards the eligible capital of Aspen Bermuda, and classified as ancillary Tier 3 capital of such entity, in accordance with applicable Bermuda laws and regulations. The total commitment under the facility is $100,000,000. A letter of credit in the full amount of the available commitment has been issued to Aspen Holdings under this facility. (viii) On December 29, 2021, Aspen Holdings entered into a committed letter of credit facility agreement. The letter of credit issued under this facility is for the benefit of Aspen Bermuda, as beneficiary, and has been applied towards the eligible capital of Aspen Bermuda, and classified as ancillary Tier 3 capital of such entity, in accordance with applicable Bermuda laws and regulations. The total commitment under the facility is $75,000,000. A letter of credit in the full amount of the available commitment has been issued to Aspen Holdings under this facility. (ix) On November 15, and 20, 2023, Aspen Bermuda and Aspen UK each signed a Global Master Repurchase Agreement with two selected banks to enable bilateral repurchase agreement to be entered, with cash and US Government Bonds as eligible collateral for the margin transfer. Advances may be used to support general corporate purposes. As of December 31, 2023, no active repurchase agreement has been entered with either of the banks. The above credit facilities include certain restrictive covenants customary for facilities of this type, including restrictions on indebtedness, consolidated tangible net worth, and minimum financial strength ratings, with such financial covenants largely consistent with these set forth in the Credit Agreement. In addition, the agreements include default covenants, which could require the Company to fully secure the outstanding amounts thereunder and/or result in the Company not being allowed to issue any new letters of credit. At December 31, 2023, no conditions of default existed under these facilities. Debt Facilities On July 26, 2023, the Company entered into a $300.0 million term loan facility at a borrowing rate of Term Secured Overnight Financing Rate (“SOFR”) plus an applicable margin, which will adjust depending on the form of the loan and long-term debt rating of Aspen Holdings, as determined by specified rating issuers from time to time. The Company drew down on the term loan on November 9, 2023 due November 9, 2026 (the “2026 Term Loan”) and the proceeds were used to settle the 2023 Senior Notes. Subject to applicable law, the 2026 Term Loan will be the senior unsecured obligations of Aspen Holdings and will rank equally in right of payment with all of our other senior unsecured indebtedness from time to time outstanding. The Company has recorded the long-term debt at amortized cost in the consolidated balance sheet. Interest incurred on the long-term debt is included within interest expense in the consolidated statement of operations. The interest expense for the twelve months ended December 31, 2023 was $3.0 million (December 31, 2022 —$Nil). The following table summarizes our contractual obligations under long-term debt as at December 31, 2023. Payments Due By Period Contractual Basis Less than 1 year 1-3 years 3-5 years More than 5 years Total ($ in millions) Long-term Debt Obligations $ — $ 300.0 $ — $ — $ 300.0

Credit Losses

Credit Losses	12 Months Ended
	Dec. 31, 2023
Credit Loss [Abstract]
Allowance for Expected Credit Losses	Allowance for Expected Credit Losses The following tables summarize the Company’s allowance for expected credit losses for the twelve months ended December 31, 2023 and December 31, 2022 in available for sale investments, reinsurance recoverables and receivables: Available for Sale Investments December 31, 2023 2022 ($ in millions) Balance at the beginning of the period $ 7.7 $ 2.7 Additions to the allowance for credit losses on securities for which credit losses were not previously recognized 0.3 5.4 Increases/(decreases) to the allowance for credit losses on securities that had an allowance in the prior period (3.6) 0.2 Reductions to the allowance for securities sold (1.5) (0.6) Balance at the end of the period $ 2.9 $ 7.7 December 31, 2023 December 31, 2022 ($ in millions) ($ in millions) Reinsurance Recoverables Receivables Reinsurance Recoverables Receivables Balance at the beginning of the year $ 3.7 $ 25.0 $ 3.3 $ 30.2 Movement in the year — (4.0) 0.4 (5.2) Balance at the end of the year $ 3.7 $ 21.0 $ 3.7 $ 25.0

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2023
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events On March 1, 2024, the Company’s Board of Directors declared the following dividends: Dividend Payable on: Record Date: 5.950% Preference Shares (AHL PRC) $ 0.3719 April 1, 2024 March 15, 2024 5.625% Preference Shares (AHL PRD) $ 0.3516 April 1, 2024 March 15, 2024 5.625% Preference Shares, represented by depositary shares (AHL PRE) (1) $ 351.56 April 1, 2024 March 15, 2024 ________________ (1) The 5.625% Preference Shares are represented by depositary shares, each representing a 1/1000 th interest in a share of the 5.625% Preference Shares. The dividend paid per depositary share is likewise 1/1000 th of the declared dividend, equivalent to $0.35156 per depositary share. The Company also paid an ordinary shares dividend of $25 million to Highlands Bermuda Holdco, Ltd. on March 21, 2024.

Schedule I - Investments

Schedule I - Investments	12 Months Ended
	Dec. 31, 2023
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Abstract]
Schedule I - Investments	Type of investment Amortized Cost or Cost Fair Value Amount at which shown in the Balance Sheet Fixed income maturities U.S. government $ 1,473.6 $ 1,448.1 $ 1,448.1 U.S. agency 7.5 7.2 7.2 Municipal 136.9 131.2 131.2 Corporate 2,229.9 2,130.8 2,130.8 High yield loans 90.8 92.1 92.1 Non-U.S. government-backed corporate 115.1 109.0 109.0 Non-U.S. government 315.7 308.6 308.6 Asset-backed 936.0 908.2 908.2 Non-agency commercial mortgage-backed 6.6 5.8 5.8 Agency mortgage-backed 544.9 467.3 467.3 Total fixed income securities $ 5,857.0 $ 5,608.3 $ 5,608.3 Short term investments $ 95.7 $ 95.7 $ 95.7 Catastrophe bonds $ 1.6 $ 1.6 $ 1.6 Privately held investments (1) $ 509.6 $ 437.1 $ 489.9 Investments, equity method $ 7.6 $ 7.6 Other investments at fair value (2) $ 169.6 $ 209.3 Total investments $ 6,319.8 $ 6,412.4 ________________ (1) Privately-held investments excludes related party investments totaling $112.4 million. (2) Other investments excludes related party investments of $23.9 million in Apollo Real Estate Fund and $15.9 million in Apollo Origination Partnership.

Schedule II - Condensed Financi

Schedule II - Condensed Financial Information of Registrant	12 Months Ended
	Dec. 31, 2023
Condensed Financial Information Disclosure [Abstract]
Schedule II - Condensed Financial Information of Registrant	SCHEDULE II - CONDENSED FINANCIAL INFORMATION OF REGISTRANT BALANCE SHEETS As at December 31, 2023 As at December 31, 2022 ($ in millions, except per share amounts) ASSETS Fixed income maturities (trading) 43.2 40.5 Cash and cash equivalents 43.5 44.4 Investments in subsidiaries (1) 3,284.2 2,767.9 Intercompany funds due from affiliates 4.3 1.7 Right-of-use operating lease assets 1.5 2.0 Other assets 5.6 5.9 Total assets $ 3,382.3 $ 2,862.4 LIABILITIES Accrued expenses and other payables 27.8 22.3 Intercompany funds due to affiliates 144.7 180.5 Long-term debt 300.0 — Short-term debt — 299.9 Operating lease liabilities 1.3 1.7 Total liabilities $ 473.8 $ 504.4 SHAREHOLDERS’ EQUITY Ordinary shares $ 0.6 $ 0.6 Preference shares 753.5 753.5 Additional paid in capital 761.2 761.2 Retained earnings 1,793.5 1,349.0 Accumulated other comprehensive income, net of taxes: Unrealized (loss) on investments (227.6) (333.2) (Loss)/gain on derivatives (0.2) 13.8 Cumulative (losses) on foreign currency translation adjustments (172.5) (186.9) Total accumulated other comprehensive (loss) (400.3) (506.3) Total shareholders’ equity 2,908.5 2,358.0 Total liabilities and shareholders’ equity $ 3,382.3 $ 2,862.4 ________________ (1) The Company’s investment in subsidiaries is accounted for under the equity method and adjustments to the carrying value of these investments are made based on the Company’s share of capital, including share of income and expenses. Changes in the value were recognized in realized and unrealized investment gains and losses in the statement of operations. Twelve Months Ended December 31, 2023 Twelve Months Ended December 31, 2022 Twelve Months Ended December 31, 2021 ($ in millions) Operating Activities: Equity in net earnings/(losses) of subsidiaries and other investments, equity method $ 304.7 $ 14.2 $ (90.5) Dividend income 364.4 121.7 193.0 Net realized and unrealized investment gains/(losses) 1.1 (4.0) (0.5) Other income 3.2 1.0 — Total revenues 673.4 132.9 102.0 Expenses: General, administrative and corporate expenses (121.3) (62.7) (54.6) Interest expense (15.6) (14.3) (14.3) Other expense (1.8) (4.8) (3.3) Income from operations before income taxes 534.7 51.1 29.8 Income tax expense — — — Net income 534.7 51.1 29.8 Other comprehensive income/(loss), net of taxes: Change in unrealized gains/(losses) on investments 105.6 (367.8) (158.6) Net change from current period hedged transactions (14.0) 15.4 (6.2) Change in foreign currency translation adjustment 14.4 (30.9) 21.4 Other comprehensive income/(loss), net of tax 106.0 (383.3) (143.4) Comprehensive Income /(loss) $ 640.7 $ (332.2) $ (113.6) Twelve Months Ended December 31, 2023 Twelve Months Ended December 31, 2022 Twelve Months Ended December 31, 2021 ($ in millions) Cash Flows From Operating Activities: Net income (excluding equity in net earnings of subsidiaries) $ 230.0 $ 36.9 $ 120.3 Adjustments: Realized and unrealized (gains)/losses (14.8) 19.7 (6.7) Loss/(gain) on derivative contracts 14.0 (15.4) 6.2 Amortization of right-of-use operating lease assets 0.5 0.5 0.5 Interest on operating lease liabilities 0.1 0.1 0.1 Change in other assets 0.3 2.1 (1.6) Change in accrued expenses and other payables 5.5 (3.2) 1.4 Change in intercompany activities (38.4) 41.1 35.0 Change in right-of use assets — — (2.0) Change in operating lease liabilities (0.5) (0.6) 1.5 Net cash provided by operating activities 196.7 81.2 154.7 Cash Flows From Investing Activities: (Purchases)/proceeds of fixed income securities (8.1) (10.0) (42.6) Proceeds from sales and maturities of fixed maturities, trading 6.4 7.8 — Investment in subsidiaries (105.7) 5.0 (115.0) Net cash (used in)/provided by investing activities (107.4) 2.8 (157.6) Cash Flows From Financing Activities: Repayment of short-term debt (300.0) — — Proceeds from term loan facility 300.0 — — Dividends paid on ordinary shares (40.3) (40.0) — Dividends paid on preference shares (49.9) (44.6) (44.5) Net cash (used in) financing activities (90.2) (84.6) (44.5) (Decrease) in cash and cash equivalents (0.9) (0.6) (47.4) Cash and cash equivalents — beginning of period 44.4 45.0 92.4 Cash and cash equivalents — end of period $ 43.5 $ 44.4 $ 45.0

Schedule III - Supplementary In

Schedule III - Supplementary Insurance Information	12 Months Ended
	Dec. 31, 2023
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Abstract]
Schedule III - Supplementary Insurance Information	SCHEDULE III - SUPPLEMENTARY INSURANCE INFORMATION Supplementary Information ($ in millions) Year Ended December 31, 2023 Deferred Policy Acquisition Costs Net Reserves for Losses and LAE Net Net Earned Premiums Net Investment Income Losses and LAE Expenses Policy Acquisition Expenses Net Written Premiums General and Administrative Expenses Reinsurance $ 201.5 $ 1,373.1 $ 644.5 $ 1,154.5 $ 611.1 $ 208.6 $ 1,098.0 $ 120.6 Insurance 94.7 1,859.7 1,048.3 1,460.0 941.9 171.6 1,483.9 233.9 Total $ 296.2 $ 3,232.8 $ 1,692.8 $ 2,614.5 $ 275.7 $ 1,553.0 $ 380.2 $ 2,581.9 $ 354.5 Year to date December 31, 2022 Deferred Policy Acquisition Costs Net Reserves for Losses and LAE Net Net Earned Premiums Net Investment Income Losses and LAE Expenses Policy Acquisition Expenses Net Written Premiums General and Administrative Expenses Reinsurance $ 227.2 $ 1,360.7 $ 862.4 $ 1,251.8 $ 770.3 $ 252.4 $ 1,426.4 $ 142.5 Insurance 91.8 1,452.5 857.8 1,436.9 909.7 179.4 1,469.6 244.0 Total $ 319.0 $ 2,813.2 $ 1,720.2 $ 2,688.7 $ 188.1 $ 1,680.0 $ 431.8 $ 2,896.0 $ 386.5 Year to date December 31, 2021 Deferred Policy Acquisition Costs Net Reserves for Losses and LAE Net Net Earned Premiums Net Investment Income Losses and LAE Expenses Policy Acquisition Expenses Net Written Premiums General and Administrative Expenses Reinsurance $ 205.2 $ 2,148.4 $ 688.6 $ 1,118.8 $ 705.2 $ 221.6 $ 1,199.0 $ 121.3 Insurance 85.6 2,165.3 827.6 1,291.7 988.1 192.5 1,388.7 211.8 Total $ 290.8 $ 4,313.7 $ 1,516.2 $ 2,410.5 $ 147.5 $ 1,693.3 $ 414.1 $ 2,587.7 $ 333.1

Schedule IV - Reinsurance

Schedule IV - Reinsurance	12 Months Ended
	Dec. 31, 2023
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Abstract]
Schedule IV - Reinsurance	SCHEDULE IV - REINSURANCE Premiums Written Direct Assumed Ceded Net Amount ($ in millions) 2023 $ 2,446.6 $ 1,521.0 $ (1,385.7) $ 2,581.9 2022 $ 2,531.7 $ 1,807.0 $ (1,442.7) $ 2,896.0 2021 $ 2,341.4 $ 1,597.0 $ (1,350.7) $ 2,587.7 Premiums Earned Gross Amount Assumed From Other Companies Ceded to Other Companies Net Amount Percentage of Amount Assumed to Net ($ in millions, except for percentages) 2023 $ 2,444.8 $ 1,562.0 $ (1,392.3) $ 2,614.5 59.7 % 2022 $ 2,370.8 $ 1,617.2 $ (1,299.3) $ 2,688.7 60.1 % 2021 $ 2,139.1 $ 1,479.2 $ (1,207.8) $ 2,410.5 61.4 %

Schedule V - Valuation and Qual

Schedule V - Valuation and Qualifying Accounts	12 Months Ended
	Dec. 31, 2023
SEC Schedule, 12-09, Valuation and Qualifying Accounts [Abstract]
Schedule V - Valuation and Qualifying Accounts	SCHEDULE V - VALUATION AND QUALIFYING ACCOUNTS The following table shows the movement in the Company’s bad debt provision during the twelve months ended December 31, 2023, 2022 and 2021: Balance at Beginning of Year Charged to Costs and Expenses Charged to Other Accounts Deductions Balance at End of Year Provisions for Bad Debt ($ in millions) 2023 Premiums receivable from underwriting activities $ 25.0 $ (4.0) $ — $ — $ 21.0 Reinsurance $ — $ — $ — $ — $ — 2022 Premiums receivable from underwriting activities $ 30.2 $ (5.2) $ — $ — $ 25.0 Reinsurance $ — $ — $ — $ — $ — 2021 Premiums receivable from underwriting activities $ 34.0 $ (3.8) $ — $ — $ 30.2 Reinsurance $ — $ — $ — $ — $ —

Basis of Preparation and Sign_2

Basis of Preparation and Significant Accounting Policies Basis of Preparation and Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2023
Accounting Policies [Abstract]
Use of Estimates	Use of Estimates
Accounting for Insurance and Reinsurance Operations	Accounting for Insurance and Reinsurance Operations Premiums Earned. Premiums are generally recorded as written on the inception date of a policy. For proportional reinsurance treaty contracts, written premiums are generally recorded as the reinsured policies attach to the treaty. For multi-year insurance or reinsurance contracts, written premiums are recorded based on the contract terms. Premiums are recognized as revenues proportionately over the coverage period. Premiums earned are recorded in the consolidated statements of operations, net of the cost of purchased reinsurance. Premiums written which are not yet recognized as earned premium are recorded in the consolidated balance sheet as unearned premiums. Written and earned premiums and the related costs include estimates for premiums which have not been finally determined. These relate mainly to contractual provisions for the payment of adjustment or additional premiums, premiums payable under proportional treaties and delegated underwriting authorities, and reinstatement premiums. Adjustment and additional premiums are premiums charged which relate to experience during the policy term. The proportion of adjustable premiums included in the premium estimates varies between business lines with the largest adjustment premiums being in property and casualty reinsurance, marine, aviation and energy insurance and the smallest in property and casualty insurance. Premiums under proportional treaty contracts and delegated underwriting authorities are generally not reported to the Company until after the reinsurance coverage is in force. As a result, an estimate of these “pipeline” premiums is recorded. The Company estimates pipeline premiums based on projections of ultimate premium taking into account reported premiums and expected development patterns. Reinstatement premiums on assumed excess of loss reinsurance contracts are provided based on experience under such contracts. Reinstatement premiums are the premiums charged for the restoration of the reinsurance limit of an excess of loss contract to its full amount after payment by the reinsurer of losses as a result of an occurrence. The original premiums are recognized as revenue in full at the date of loss, with the reinstatement premiums recognized as revenue over the remaining cover term. Reinstatement premiums provide future insurance cover for the remainder of the initial policy term. An allowance for uncollectible premiums is established for possible non-payment of premium receivables, as deemed necessary. Credit Losses on Underwriting Premiums Receivable. Underwriting premium receivable balances are reported net of an allowance for expected credit losses. The allowance, based on ongoing review and monitoring of amounts outstanding, historical loss data, including write-offs and other current economic factors, is charged to net income in the period the receivable is recorded and revised in subsequent periods to reflect changes in the Company’s estimate of expected credit losses. For most insurance policies, credit risk is partially mitigated by the Company’s ability to cancel the policy if the policyholder does not pay the premium whereby, upon default, policy liabilities would be written-down along with premium receivables. Losses and Loss Adjustment Expenses. Losses represent the amount paid or expected to be paid to claimants in respect of events that have occurred on or before the balance sheet date. The costs of investigating, resolving and processing these claims are known as loss adjustment expenses (“LAE”). The consolidated statements of operations records these losses net of reinsurance, meaning that gross losses and loss adjustment expenses incurred are reduced by the amounts recovered or expected to be recovered under reinsurance contracts. Reinsurance. Written premiums, earned premiums, incurred claims, LAE and the acquisition costs all reflect the net effect of assumed and ceded reinsurance transactions. Assumed reinsurance refers to the Company’s acceptance of certain insurance risks that other insurance companies have underwritten. Ceded reinsurance arises from contracts under which other insurance companies agree to share certain risks with the Company. Reinsurance accounting is followed when there is significant timing risk, underwriting risk and transfer risk, and a reasonable possibility of significant loss. Outward reinsurance premiums, which are paid when the Company purchases reinsurance or retrocessional coverage, are accounted for using the same accounting methodology as the Company uses for inwards premiums. Premiums payable under reinsurance contracts that operate on a “losses occurring during” basis are expensed over the period of coverage while those arising from “risks attaching during” policies are expensed over the earnings period of the underlying premiums written from the reinsured business. Adjustment premiums and reinstatement premiums in relation to outward reinsurance are accrued when it is determined that the ultimate losses will trigger a payment and recognized within premiums payable. Reinsurance and retrocession does not isolate the ceding company from its obligations to policyholders. In the event that a reinsurer or retrocessionaire fails to meet its obligations, the ceding company’s obligations remain. Accounting for Retroactive Reinsurance Agreements. Retroactive reinsurance agreements are reinsurance agreements under which a reinsurer agrees to reimburse the Company as a result of past insurable events. For retroactive reinsurance purchased by the Company, the excess of the amounts ultimately collectible under the agreement over the consideration paid is recognized as a deferred gain liability which is amortized into income over the settlement period of the ceded reserves once the paid losses have exceeded the minimum retention. The amount of the deferral is recalculated each period based on actual loss payments and updated estimates of ultimate losses. If the consideration paid exceeds the ultimate losses collectible under the agreement, the net loss on the retroactive reinsurance agreement is recognized within income immediately. Premiums payable for retroactive reinsurance coverage and meeting the conditions of reinsurance accounting are reported as reinsurance recoverables to the extent that those amounts do not exceed recorded liabilities relating to underlying reinsurance contracts. Premiums paid in excess of accounts receivable are charged to income. Reserves. Insurance reserves are established for the total unpaid cost of claims and LAE in respect of events that have occurred by the balance sheet date, including the Company’s estimates of the total cost of claims incurred but not yet reported (“IBNR”). Claim reserves are reduced for estimated amounts of salvage and subrogation recoveries. Estimated amounts recoverable from reinsurers on unpaid losses and LAE are reflected as assets. For reported claims, reserves are established on a case-by-case basis within the parameters of coverage provided in the insurance policy or reinsurance agreement. For IBNR claims, reserves are estimated using a number of established actuarial methods to establish a range of estimates from which a management best estimate is selected. Both case and IBNR reserve estimates consider variables such as past loss experience, changes in legislative conditions, changes in judicial interpretation of legal liability, policy coverages and inflation. As many of the coverages underwritten involve claims that may not be ultimately settled for many years after they are incurred, subjective judgments as to the ultimate exposure to losses are an integral and necessary component of the loss reserving process. The Company regularly reviews its reserves, using a variety of statistical and actuarial techniques to analyze current claims costs, frequency and severity data, and prevailing economic, social and legal factors. Reserves established in prior periods are adjusted as claim experience develops and new information becomes available. Adjustments to previously estimated reserves are reflected in the financial results of the period in which the adjustments are made. The process of estimating required reserves does, by its very nature, involve considerable uncertainty. The level of uncertainty can be influenced by factors such as the existence of coverage with long duration payment patterns and changes in claims handling practices, as well as the factors noted above. Ultimate actual payments for claims and LAE could turn out to be significantly different from the Company’s estimates. Credit Losses on Reinsurance Recoverables. Amounts recoverable from reinsurers are estimated in a manner consistent with the claim liability with the reinsured business. The Company maintains credit risk if a reinsurer is unable to pay recoverables when they become due. To manage this risk, the Company evaluates the financial condition of its reinsurers and retrocessionaires, and monitors concentration of credit risk to minimize its exposure to significant losses from individual reinsurers. To further reduce credit exposure on reinsurance recoverables, the Company has received collateral, including letters of credit and trust accounts, from certain reinsurers. Following the adoption of ASC 326, an allowance is established for expected credit losses to be recognized over the life of the reinsurance recoverable. The allowance considers the current financial strength of the individual reinsurer and the amount of collateral held. Acquisition Costs. The costs directly related to writing a (re)insurance policy are referred to as acquisition expenses and include commissions, premium taxes and profit commissions. With the exception of profit commissions, these expenses are incurred when a policy is issued, and only the costs directly related to the successful acquisition of new and renewal insurance and reinsurance contracts are deferred and amortized over the same period as the corresponding premiums are recorded as revenues. Profit commissions are estimated and accrued based on the related performance criteria evaluated at the balance sheet date, with subsequent changes to those estimates recognized when they occur. Commissions received related to reinsurance premiums ceded are netted against broker commissions in determining acquisition costs eligible for deferral. On a regular basis a premium deficiency analysis is performed of the deferred acquisition costs in relation to the expected recognition of revenues, including anticipated investment income, and adjustments, if any, are reflected as period costs. Should the analysis indicate that the acquisition costs are unrecoverable, further analyses are performed to determine if a reserve is required to provide for losses which may exceed the related unearned premium. General and Administrative Expenses. These costs represent the expenses incurred in running the business and include, but are not limited to compensation costs for employees, rental costs, IT development and professional and consultancy fees. General and administrative costs directly attributable to the successful acquisition of business are deferred and amortized over the same period as the corresponding premiums are recorded as revenues. When reporting the results for its business segments, the Company includes expenses which are directly attributable to the segment plus an allocation of central administrative costs. Corporate Expenses.
Accounting for Investments, Cash and Cash Equivalents	Accounting for Investments, Cash and Cash Equivalents Fixed Income Securities. The fixed income securities portfolio comprises securities issued by governments and government agencies, corporate bonds, mortgage and other asset-backed securities and bank loans. Investments in fixed income securities are classified as available for sale or trading and are reported at estimated fair value in the consolidated balance sheet. Investment transactions are recorded on the trade date with balances pending settlement reflected in the consolidated balance sheet under receivables for securities sold and other payables for securities purchased, respectively. Fair values are based on quoted market prices and other data provided by third-party pricing services. Short-term Investments. Short-term investments primarily comprise highly liquid debt securities with a maturity greater than three months but less than one year from the date of purchase and are held as part of the investment portfolio of the Company. Short-term investments are classified as either trading or available for sale and reported at estimated fair value. Catastrophe Bonds. Investments in catastrophe bonds are classified as trading and are reported on the consolidated balance sheet at estimated fair value. The fair values are based on independent broker-dealer quotes. Privately-held Investments. The Company’s privately-held investments primarily comprise commercial mortgage loans, middle market loans and other private debt, asset-backed securities and global corporate securities. These investments are classified as trading or available for sale and are reported on the consolidated balance sheet at estimated fair value. Privately-held investments are initially valued at cost or transaction value which approximates fair value. In subsequent measurement periods, the fair values of these securities are primarily determined using discounted cash flow models. Interest income is accrued on the principal amount of the loan based on its contractual interest rate subject to it being probable that we will receive interest on that particular underlying loan. Interest income, amortization of premiums and discounts, and prepayment fees are reported in net investment income on the consolidated statements of income. Other Investments, Equity Method. Other investments represent the Company’s investments that are recorded using the equity method of accounting. Adjustments to the fair value of these investments are made based on the net asset value of the investment. Other Investments. Other investments represent the Company’s investments in investment funds that are reported at fair value. Adjustments to the fair value are made based on the net asset value of the investment. Cash and Cash Equivalents. Cash and cash equivalents are reported at fair value. Cash and cash equivalents comprise cash on hand, deposits held on call with banks and other short-term highly liquid investments due to mature within three months from the date of purchase and which are subject to insignificant risk of change in fair value. Gains and Losses. Realized gains or losses on the sale of investments are determined on the basis of the first in first out cost method and are recorded in revenue or expenses respectively. Unrealized gains and losses represent the difference between the cost, or the cost as adjusted by amortization of any difference between its cost and its redemption value (“amortized cost”), of the security and its fair value at the reporting date and are included within other comprehensive income for securities classified as available for sale and in realized and unrealized investment gains or losses in the consolidated statement of operations for securities classified as trading. Credit Losses on Available for Sale Debt Securities . An allowance account for credit losses is recognized for available for sale debt securities based on a review of individual securities. Write-offs are recorded when amounts are deemed uncollectible, or Aspen intends to sell (or more likely than not will be required to sell) the debt security before recovery of the amortized cost basis. The amortized cost basis will be written down to the debt securities fair value through earnings. Credit losses are limited to the difference between the debt securities amortized cost basis and fair value (‘fair-value floor’). Any decline in the debt securities fair value below the amortized cost basis that is not a result of a credit loss is recorded through other comprehensive income, net of applicable taxes. The allowance for credit losses of a security may be increased or reversed upon a change in credit position with the change reflected in net income. The credit loss models employ a discounted cash flow approach to evaluate whether a credit loss exists at the individual security level and are reviewed at each reporting period. This analysis excludes investments in U.S. Government / Agency bonds and U.S. Government Agency mortgage-backed securities due to being of ‘high credit quality’ based on the absence of risk. For any available for sale debt securities that were initially purchased with credit deterioration (PCD), the amortized cost basis shall be considered to be the purchase price, plus any allowance for credit losses. Estimated credit losses shall be discounted at the rate that equates the present value of the purchaser’s estimate of the security’s future cash flows with the purchase price of the asset. Net Investment Income.
Accounting for Derivative Financial Instruments	Accounting for Derivative Financial Instruments The Company enters into derivative instruments to manage certain market risks, such as forward exchange contracts used to reduce foreign currency risk relative to the U.S. dollar. The Company records derivative instruments at fair value on the Company’s consolidated balance sheet as either assets or liabilities, depending on their rights and obligations. The accounting for the gain or loss due to the changes in the fair value of these instruments is dependent on whether the derivative qualifies as a hedge. If the derivative does not qualify as a hedge, the gains or losses are reported in the consolidated statement of operations when they occur and classified within Change in fair value of derivatives . If the derivative qualifies as a hedge, the accounting treatment varies based on the type of risk being hedged. There are two primary types of hedging relationships that may be used for accounting purposes: fair value hedge and cash flow hedge. A fair value hedge is designed to offset changes in the fair value of an underlying asset or liability, and the gain or loss from the hedging instrument offsets the change in fair value of the underlying asset or liability. Under fair value hedge accounting, both the gain or loss from the underlying asset or liability and the gain or loss from the hedging instrument are recognized in earnings in the same period. In contrast, a cash flow hedge is designed to offset changes in cash flows of an underlying asset or liability. The gain or loss from the hedging instrument is initially recognized in other comprehensive income. As the contracts settle, the realized gain or loss is reclassified from other comprehensive income into the consolidated statement of operations. The loss portfolio transfer contract includes a funds withheld arrangement that provides a variable interest expense based on Aspen’s investment performance. As a result, this funds withheld arrangement is considered an embedded derivative and accounted for as an option-based derivative. Since the economic characteristics and risks of an embedded derivative feature are not clearly and closely related to the economic characteristics and risks of the host contract, the embedded derivative is bifurcated and accounted for separately at fair value. The Company records subsequent changes in the embedded derivative fair value in the consolidated statement of operations.
Accounting for Intangible Assets	Accounting for Intangible Assets Intangible assets are held in the consolidated balance sheet at cost less amortization and impairment. Amortization applies on a straight-line basis in respect of assets having a finite estimated useful economic life. Finite intangibles are assessed on an annual basis for impairment, or more frequently where circumstances indicate the carrying value may not be recoverable. For intangible assets considered to have an indefinite life, the Company performs a qualitative assessment annually to determine whether it is more likely than not that an intangible asset is impaired. Goodwill is assessed annually for impairment or more frequently if circumstances indicate an impairment may have occurred.
Accounting for Office Properties and Equipment	Accounting for Office Properties and Equipment Office properties and equipment are reported at cost less accumulated depreciation. These assets are depreciated on a straight-line basis over the estimated useful lives of the assets. Computer equipment is depreciated between three IT development costs that are directly associated with the development of identifiable and unique software products and that are anticipated to generate economic benefits exceeding costs beyond one year, are recognized within office properties and equipment. Costs include external consultants’ fees, certain qualifying internal staff costs and other costs incurred to develop software programs. Software is depreciated over their estimated useful life, between three
Accounting for Right-of-Use Operating Lease Assets	Accounting for Leases In the ordinary course of the business, the Company renews and enters into new leases for office real estate and other assets. At the lease inception date, the Company determines whether a contract contains a lease and recognizes operating lease Right-of-use assets and operating lease liabilities based on the present value of future minimum lease payments. As our leases do not provide an implicit interest rate, we use our incremental borrowing rate based on the information available at the commencement date to determine the present value of future payments. For all office real estate leases, rent incentives, including reduced-rent and rent free periods and contractually agreed rent increases during the lease term, have been included when determining the present value of future cash flows. Right-of-use operating lease assets are reported at cost less accumulated depreciation on the consolidated balance sheet and depreciated over the lease term. The Company does not record office property and equipment leases with an initial term of 12 months or less (short-term) in the Company's consolidated balance sheets. Such short-term leases are expensed through the consolidated statement of operations.
Accounting for Foreign Currencies Translation	Accounting for Foreign Currencies Translation The functional currency of the Company and its subsidiaries is the U.S. Dollar, which is also the Company’s reporting currency. Transactions in currencies other than the functional currency are measured at the exchange rate prevailing at the date of the transaction. Monetary assets and liabilities denominated in non-functional currencies are remeasured at the exchange rate prevailing at the balance sheet date and any resulting foreign exchange gains or losses are reflected in the consolidated statement of operations. Foreign exchange gains or losses related to available for sale investments denominated in non-functional currencies are included within other comprehensive income. Non-monetary assets and liabilities are remeasured to functional currency at historic exchange rates.
Accounting for Income Tax	Accounting for Income Taxes Income taxes are accounted for under the asset and liability method. Deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases and for operating loss and tax credit carryforwards. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. When the Company does not believe that, on the basis of available information, it is more likely than not that deferred tax assets will be fully recovered, it recognizes a valuation allowance against its deferred tax assets to reduce the deferred tax assets to the amount more likely than not to be realized. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that includes the enactment date. Furthermore, a tax benefit from a tax position may be recognized in the financial statements only if it is more-likely-than-not that the position is sustainable, based solely on its technical merits and consideration of the relevant tax authority’s widely understood administrative practices and precedents. The Company applies a portfolio approach to release the income tax effects in accumulated other comprehensive income. Under this approach, the income tax effects upon the sale of an available for sale debt security, settlement of hedged transactions and upon foreign currency translation adjustments for each period, are determined under the intraperiod tax allocation approach. Any tax effects remaining in accumulated other comprehensive income are only released when the entire portfolio is liquidated, sold or extinguished.
Accounting for Preference Shares	Accounting for Preference Shares
Accounting for Long-term Incentive Plans	Accounting for Share-Based Payments and Long-Term Incentive Plans The Company operates an employee long-term incentive plan, comprised of Performance Units and Exit Units, the terms and conditions of which are described in Note 17. The Company applies a fair-value based measurement method in calculating the compensation costs of Performance Units which are recognized on a straight-line basis over the vesting period. Certain employees of the Company participate in a Management Equity Plan (“MEP”), comprising stock options to acquire non-voting shares in a related party affiliate of the Company, at no cost to the employee. The terms and conditions of the MEP are described in Note 17. The Company recognizes compensation costs for these awards with performance conditions if, and when, the Company concludes that it is probable that the performance condition(s) will be achieved, over the requisite service period. The Company reviews and evaluates these estimates regularly and will recognise any remaining unrecognised compensation expense as an estimate revision. The stock options vest upon a number of performance conditions; an exit or liquidity event occurring; a two-year cumulative operating income hurdle being achieved over the vesting period; and certain other contractual terms being achieved. The Company applies fair-value based measurement principles to determine grant date values for the stock options. The Company has made an entity-wide accounting election to account for award forfeitures as they occur.
Accounting for Business Combinations	Accounting for Business Combinations The Company accounts for a transaction as a business combination where the assets acquired and liabilities assumed following a transaction constitute a business. An acquired entity must have inputs and processes that make it capable of generating a return or economic benefit to be considered a business. If the assets acquired are not a business, the Company accounts the transaction as an asset acquisition. The Company recognizes and measures at fair value 100 percent of the assets and liabilities of any acquired business. Goodwill is recognized and measured as the difference between the consideration paid or payable less the fair value of assets acquired. The Company accounts for the disposal of subsidiary undertakings when it ceases to control the subsidiary’s assets and liabilities or the group of assets. A gain or loss is recognized and measured as the difference between the fair value of consideration received or receivable and the value of assets, liabilities and equity components de-recognized, related to that subsidiary or group of assets when deconsolidated. Costs that are directly related to a business combination transaction are expensed in the periods in which the costs are incurred and the services are received.
Accounting Pronouncements	Accounting Pronouncements Accounting Pronouncements Not Yet Adopted Segment Reporting In November 2023, the FASB issued Accounting Standards Update (“ASU”) No. 2023-07, “Segment Reporting (Topic 280) - Improvements to Reportable Segment Disclosures” . This update improves the disclosures about a public entity’s reportable segments and addresses requests from investors for additional, more detailed information about a reportable segment’s expenses. The amendments in this ASU are effective for fiscal years beginning after December 15, 2023, and interim periods within fiscal years beginning after December 15, 2024. As this guidance relates solely to financial statement disclosures, the adoption of ASU 2023-07 will have no impact upon the Company’s results of operations, financial condition, or liquidity. Income Taxes In December 2023, the FASB issued ASU 2023-09, “Income Taxes (Topic 740) - Improvements to Income Tax Disclosures” . The amendments in this Update provide additional transparency about income tax information through improvements to income tax disclosures primarily related to the rate reconciliation and income taxes paid information. The amendments to this Update are effective for annual periods beginning after December 15, 2024. As this guidance relates solely to financial statement disclosures, the adoption of ASU 2023-09 will have no impact upon the Company’s results of operations, financial condition, or liquidity. Other accounting pronouncements were issued during the year ended December 31, 2023 which were either not applicable to the Company or did not impact the Company’s consolidated financial statements.

Segment Reporting (Tables)

Segment Reporting (Tables)	12 Months Ended
	Dec. 31, 2023
Segment Reporting [Abstract]
Summary of gross and net written and earned premiums, underwriting results, ratios and reserves for each of Company's business segments	Twelve Months Ended December 31, 2023 Reinsurance Insurance Total ($ in millions) Underwriting Revenues Gross written premiums $ 1,521.0 $ 2,446.6 $ 3,967.6 Net written premiums 1,098.0 1,483.9 2,581.9 Gross earned premiums 1,562.0 2,444.8 4,006.8 Net earned premiums 1,154.5 1,460.0 2,614.5 Underwriting Expenses Losses and loss adjustment expenses 611.1 941.9 1,553.0 Acquisition costs 208.6 171.6 380.2 General and administrative expenses 120.6 233.9 354.5 Underwriting income 214.2 112.6 326.8 Corporate and other expenses (1) (114.0) Non-operating expenses (2) (35.1) Net investment income 275.7 Realized and unrealized investment gains 75.9 Realized and unrealized investment losses (61.4) Change in fair value of derivatives 26.1 Interest expense (55.2) Net realized and unrealized foreign exchange (losses) (36.2) Income before income taxes 402.6 Income tax benefit 132.1 Net income $ 534.7 Net reserves for loss and loss adjustment expenses $ 1,373.1 $ 1,859.7 $ 3,232.8 Ratios Loss ratio 52.9 % 64.5 % 59.4 % Acquisition cost ratio 18.1 11.8 14.5 General and administrative expense ratio 10.4 16.0 13.6 Expense ratio 28.5 27.8 28.1 Combined ratio 81.4 % 92.3 % 87.5 % ________________ (1) Corporate and other expenses includes other income/(expenses), which were previously presented separately. (2) Non-operating expenses in the twelve months ended December 31, 2023 includes expenses in relation to consulting fees, non-recurring transformation program costs, and other non-recurring costs. Twelve Months Ended December 31, 2022 Reinsurance Insurance Total ($ in millions) Underwriting Revenues Gross written premiums $ 1,807.0 $ 2,531.7 $ 4,338.7 Net written premiums 1,426.4 1,469.6 2,896.0 Gross earned premiums 1,617.2 2,370.8 3,988.0 Net earned premiums 1,251.8 1,436.9 2,688.7 Underwriting Expenses Losses and loss adjustment expenses 770.3 909.7 1,680.0 Acquisition costs 252.4 179.4 431.8 General and administrative expenses 142.5 244.0 386.5 Underwriting income 86.6 103.8 190.4 Corporate expenses (71.7) Non-operating expenses (1) (36.0) Net investment income 188.1 Realized and unrealized investment gains 5.0 Realized and unrealized investment losses (182.6) Change in fair value of derivatives (80.5) Interest expense (43.7) Net realized and unrealized foreign exchange gains 15.9 Other income 8.2 Other expenses (20.1) (Loss) before income taxes (27.0) Income tax benefit 78.1 Net income $ 51.1 Net reserves for loss and loss adjustment expenses $ 1,360.7 $ 1,452.5 $ 2,813.2 Ratios Loss ratio 61.5 % 63.3 % 62.5 % Acquisition cost ratio 20.2 12.5 16.1 General and administrative expense ratio 11.4 17.0 14.4 Expense ratio 31.6 29.5 30.5 Combined ratio 93.1 % 92.8 % 93.0 % ________________ (1) Non-operating expenses in the twelve months ended December 31, 2022 includes expenses in relation to consulting fees, non-recurring transformation activities, and other non-recurring costs. Twelve Months Ended December 31, 2021 Reinsurance Insurance Total ($ in millions) Underwriting Revenues Gross written premiums $ 1,597.0 $ 2,341.4 $ 3,938.4 Net written premiums 1,199.0 1,388.7 2,587.7 Gross earned premiums 1,479.2 2,139.1 3,618.3 Net earned premiums 1,118.8 1,291.7 2,410.5 Underwriting Expenses Losses and loss adjustment expenses 705.2 988.1 1,693.3 Acquisition costs 221.6 192.5 414.1 General and administrative expenses 121.3 211.8 333.1 Underwriting income/(loss) 70.7 (100.7) (30.0) Corporate expenses (64.3) Non-operating expenses (1) (20.6) Net investment income 147.5 Realized and unrealized investment gains 56.2 Realized and unrealized investment losses (47.4) Change in fair value of derivatives (35.9) Interest expense (14.3) Net realized and unrealized foreign exchange gains 40.0 Other income 14.7 Other expenses (10.8) Income before income taxes 35.1 Income tax (expense) (5.3) Net income $ 29.8 Net reserves for loss and loss adjustment expenses $ 2,148.4 $ 2,165.3 $ 4,313.7 Ratios Loss ratio 63.0 % 76.5 % 70.2 % Acquisition cost ratio 19.8 14.9 17.2 General and administrative expense ratio 10.8 16.4 13.8 Expense ratio 30.6 31.3 31.0 Combined ratio 93.6 % 107.8 % 101.2 % ________________ (1) Non-operating expenses in the twelve months ended December 31, 2021 includes expenses in relation to consulting fees, non-recurring transformation activities, and other non-recurring costs.
Schedule of gross written premiums based on geographical areas	For the Twelve Months Ended December 31, 2023 December 31, 2022 December 31, 2021 ($ in millions) Australia/Asia $ 177.8 $ 257.5 $ 275.8 Europe 179.4 194.5 140.6 United Kingdom & Ireland 532.5 485.8 393.2 United States & Canada (1) 2,472.0 2,715.7 2,301.8 Worldwide excluding United States (2) 28.8 24.2 31.5 Worldwide including United States (3) 417.2 541.7 592.2 Other (4) 159.9 119.3 203.3 Total $ 3,967.6 $ 4,338.7 $ 3,938.4 ________________

Investments (Tables)

Investments (Tables)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Investments, Debt and Equity Securities [Abstract]
Summary of Investment Income		For the Twelve Months Ended December 31, 2023 December 31, 2022 December 31, 2021 ($ in millions) Fixed income securities — Available for sale $ 115.7 $ 95.6 $ 87.2 Fixed income securities — Trading 98.9 57.0 30.2 Short-term investments — Available for sale 5.3 0.6 0.1 Short-term investments — Trading 0.3 0.1 — Fixed term deposits (included in cash and cash equivalents) 39.9 6.6 0.7 Catastrophe bonds — Trading 0.2 0.3 0.9 Privately-held investments — Available for sale 0.1 — — Privately-held investments — Trading 44.7 24.3 18.2 Other investments, at fair value (1) (17.8) 13.9 21.9 Total 287.3 198.4 159.2 Investment expenses (11.6) (10.3) (11.7) Net investment income $ 275.7 $ 188.1 $ 147.5 ________________ (1)
Net Realized and Unrealized Investment Gains and Losses and Change in Unrealized Gains and Losses on Investments		For the Twelve Months Ended December 31, 2023 December 31, 2022 December 31, 2021 ($ in millions) Available for sale: Fixed income securities — gross realized gains $ 1.6 $ 2.9 $ 22.7 Fixed income securities — gross realized (losses) (41.5) (58.4) (3.6) Short-term investments — gross realized gains 0.6 1.0 2.0 Short-term investments — gross realized (losses) (0.9) (0.5) (0.8) Net change in expected credit gains/(losses) 4.8 (5.0) (2.5) Trading: Fixed income securities — gross realized gains 1.0 0.2 12.2 Fixed income securities — gross realized (losses) (3.5) (1.8) (2.0) Short-term investments — gross realized gains 0.1 — 0.1 Short-term investments — gross realized (losses) (0.3) — (0.3) Privately-held investments — gross realized gains 0.8 0.7 0.6 Privately-held investments — gross realized (losses) — (0.1) (13.8) Privately-held investments — net unrealized (losses)/gains (15.2) (2.5) 18.1 Catastrophe bonds — net unrealized gains/(losses) 0.1 0.2 (0.8) Fixed income securities — net unrealized gains/(losses) 65.9 (113.9) (23.4) Investments — equity method: Gross realized and unrealized gains in MVI 0.2 — 0.1 Gross unrealized gains/(losses) in Multi-Line Reinsurer 0.8 (0.4) 0.2 Total net realized and unrealized investment gains/(losses) recorded in the consolidated statement of operations $ 14.5 $ (177.6) $ 8.8 Change in available for sale net unrealized gains/(losses): Available for sale investments $ 126.2 $ (391.7) $ (157.6) Income tax (expense)/benefit (20.6) 23.9 (0.3) Total change in net unrealized gains/(losses), net of taxes recorded in other comprehensive income $ 105.6 $ (367.8) $ (157.9)
Cost or Amortized Cost, Unrealized and Realized Gains and Losses and Estimated Fair Value of Available for Sale Investments		As at December 31, 2023 Cost or Gross Gross Allowance for Credit Losses (1) Fair Market ($ in millions) Fixed Income Securities — Available for sale U.S. government $ 1,224.9 $ 4.4 $ (26.7) $ — $ 1,202.6 U.S. agency 7.5 — (0.3) — 7.2 Municipal 133.6 — (5.0) (0.5) 128.1 Corporate 2,051.1 12.1 (101.5) (2.4) 1,959.3 Non-U.S. government-backed corporate 106.5 0.1 (5.9) — 100.7 Non-U.S. government 279.9 0.6 (6.7) — 273.8 Agency commercial mortgage-backed 6.6 — (0.8) — 5.8 Agency residential mortgage-backed 519.9 0.1 (74.9) — 445.1 Total fixed income securities — Available for sale 4,330.0 17.3 (221.8) (2.9) 4,122.6 Short-term investments — Available for sale 93.6 — — — 93.6 Privately-held investments — Available for sale Asset-backed securities 14.7 0.2 — — 14.9 Total Investments — Available for sale $ 4,438.3 $ 17.5 $ (221.8) $ (2.9) $ 4,231.1 ________________ (1) For more information on the allowance for expected credit losses, refer to Note 25, “Allowance for Expected Credit Losses”. As at December 31, 2022 Cost or Gross Gross Allowance for Credit Losses Fair Market ($ in millions) Fixed Income Securities — Available for sale U.S. government $ 1,003.7 $ — $ (50.7) $ — $ 953.0 U.S. agency 9.2 — (0.4) — 8.8 Municipal 159.9 — (9.4) (1.0) 149.5 Corporate 2,016.9 3.5 (169.1) (6.3) 1,845.0 Non-U.S. government-backed corporate 119.4 — (8.8) (0.2) 110.4 Non-U.S. government 225.2 0.1 (11.5) (0.2) 213.6 Non-agency commercial mortgage-backed 6.6 — (1.0) — 5.6 Agency mortgage-backed 590.4 — (87.7) — 502.7 Total fixed income securities — Available for sale 4,131.3 3.6 (338.6) (7.7) 3,788.6 Short-term investments — Available for sale 52.4 — (0.4) — 52.0 Total Investments — Available for sale $ 4,183.7 $ 3.6 $ (339.0) $ (7.7) $ 3,840.6
Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Trading Investments in Fixed Income Maturities	As at December 31, 2023 Cost or Gross Gross Fair Market ($ in millions) Fixed Income Securities — Trading U.S. government $ 248.7 $ 0.5 $ (3.7) $ 245.5 Municipal 3.3 — (0.2) 3.1 Corporate 178.8 0.7 (8.0) 171.5 High yield loans 90.8 1.3 — 92.1 Non-U.S. government-backed corporate 8.6 — (0.3) 8.3 Non-U.S. government 35.8 0.1 (1.1) 34.8 Asset-backed 936.0 2.1 (29.9) 908.2 Agency mortgage-backed 25.0 — (2.8) 22.2 Total fixed income securities — Trading 1,527.0 4.7 (46.0) 1,485.7 Short-term investments — Trading 2.1 — — 2.1 Catastrophe bonds — Trading 1.6 — — 1.6 Privately-held investments — Tradin g Commercial mortgage loans 293.2 1.0 (19.3) 274.9 Middle market loans and other private debt 85.9 — (1.1) 84.8 Asset-backed securities 83.1 0.4 (0.6) 82.9 Global corporate securities 14.7 — (0.3) 14.4 Short-term investments 18.0 — — 18.0 Total privately-held investments — Trading 494.9 1.4 (21.3) 475.0 Total Investments — Trading $ 2,025.6 $ 6.1 $ (67.3) $ 1,964.4 As at December 31, 2022 Cost or Gross Gross Fair Market ($ in millions) Fixed Income Securities — Trading U.S. government $ 267.9 $ — $ (6.3) $ 261.6 Municipal 3.9 — (0.3) 3.6 Corporate 175.3 0.3 (13.5) 162.1 High yield loans 90.2 0.2 (2.1) 88.3 Non-U.S. government-backed corporate 12.2 — (0.6) 11.6 Non-U.S. government 32.2 — (1.8) 30.4 Asset-backed 970.3 0.2 (74.0) 896.5 Agency mortgage-backed 24.7 — (3.3) 21.4 Total fixed income securities — Trading 1,576.7 0.7 (101.9) 1,475.5 Short-term investments — Trading 6.3 — — 6.3 Catastrophe bonds — Trading 5.1 — (2.2) 2.9 Privately-held investments — Trading Commercial mortgage loans 312.6 0.6 (1.1) 312.1 Middle market loans and other private debt 109.0 — (2.1) 106.9 Asset-backed securities 68.8 — (2.0) 66.8 Global corporate securities 15.1 — (0.1) 15.0 Equity securities 6.6 — — 6.6 Short-term investments 25.6 — — 25.6 Total privately-held investments — Trading 537.7 0.6 (5.3) 533.0 Total Investments — Trading $ 2,125.8 $ 1.3 $ (109.4) $ 2,017.7
Schedule of Commercial Mortgage and Middle Market Loans

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2023
Fair Value Disclosures [Abstract]
Financial Assets Measured on Recurring Basis	As at December 31, 2023 Level 1 Level 2 Level 3 Total ($ in millions) Available for sale financial assets, at fair value U.S. government $ 1,202.6 $ — $ — $ 1,202.6 U.S. agency — 7.2 — 7.2 Municipal — 128.1 — 128.1 Corporate — 1,959.3 — 1,959.3 Non-U.S. government-backed corporate — 100.7 — 100.7 Non-U.S. government 200.4 73.4 — 273.8 Agency commercial mortgage-backed — 5.8 — 5.8 Agency residential mortgage-backed — 445.1 — 445.1 Total fixed income securities available for sale, at fair value 1,403.0 2,719.6 — 4,122.6 Short-term investments available for sale, at fair value 86.7 6.9 — 93.6 Privately-held investments available for sale, at fair value — — 14.9 14.9 Held for trading financial assets, at fair value U.S. government 245.5 — — 245.5 Municipal — 3.1 — 3.1 Corporate — 171.5 — 171.5 Non-U.S. government-backed corporate — 8.3 — 8.3 High yield loans — 92.1 — 92.1 Non-U.S. government 13.0 21.8 — 34.8 Asset-backed — 908.2 — 908.2 Agency mortgage-backed — 22.2 — 22.2 Total fixed income securities trading, at fair value 258.5 1,227.2 — 1,485.7 Short-term investments trading, at fair value 0.2 1.9 — 2.1 Catastrophe bonds trading, at fair value — 1.6 — 1.6 Privately-held investments trading, at fair value — — 475.0 475.0 Other investments (1) — — — 209.3 Other financial assets and liabilities, at fair value Derivative assets — foreign exchange contracts — 31.7 — 31.7 Derivative liabilities — foreign exchange contracts — (9.3) — (9.3) Derivative liabilities — loss portfolio transfer (2) — (16.5) (16.5) Total 1,748.4 3,979.6 473.4 6,410.7 ________________ (1) Other investments represents our investments in investment funds operating strategies in real estate, infrastructure and lending and are measured at fair value using the net asset value per share practical expedient. As a result the investments are not classified in the fair value hierarchy. The fair value amounts presented in the table above are intended to permit reconciliation of the fair value hierarchy to the amounts presented in the consolidated balance sheets. The investment in the funds is subject to restrictions as detailed in Note 21(a), “Commitments and Contingent Liabilities.” (2) The loss portfolio transfer contract includes a funds withheld arrangement that provides variable interest expense based on Aspen’s investment performance. As a result, the funds withheld arrangement is considered an embedded derivative and accounted for as an option-based derivative. At December 31, 2022 Level 1 Level 2 Level 3 Total ($ in millions) Available for sale financial assets, at fair value U.S. government $ 953.0 $ — $ — $ 953.0 U.S. agency — 8.8 — 8.8 Municipal — 149.5 — 149.5 Corporate — 1,845.0 — 1,845.0 Non-U.S. government-backed corporate — 110.4 — 110.4 Non-U.S. government 145.3 68.3 — 213.6 Non-agency commercial mortgage-backed — 5.6 — 5.6 Agency mortgage-backed — 502.7 — 502.7 Total fixed income securities available for sale, at fair value 1,098.3 2,690.3 — 3,788.6 Short-term investments available for sale, at fair value 51.9 0.1 — 52.0 Held for trading financial assets, at fair value U.S. government 261.6 — — 261.6 Municipal — 3.6 — 3.6 Corporate — 162.1 — 162.1 Non-U.S. government-backed corporate — 11.6 — 11.6 High yield loans — 88.3 — 88.3 Non-U.S. government 10.6 19.8 — 30.4 Asset-backed — 896.5 — 896.5 Agency mortgage-backed — 21.4 — 21.4 Total fixed income securities trading, at fair value 272.2 1,203.3 — 1,475.5 Short-term investments trading, at fair value 6.3 — — 6.3 Catastrophe bonds trading, at fair value — 2.9 — 2.9 Privately-held investments — — 533.0 533.0 Other investments (1) — — — 221.3 Other financial assets and liabilities, at fair value Derivative assets — foreign exchange contracts — 56.2 — 56.2 Derivative liabilities — foreign exchange contracts — (3.2) — (3.2) Derivative liabilities — loss portfolio transfer (2) — — (31.7) (31.7) Total $ 1,428.7 $ 3,949.6 $ 501.3 $ 6,100.9 ________________ (1) Other investments represents our investments in investment funds operating strategies in real estate, infrastructure and lending and are measured at fair value using the net asset value per share practical expedient. As a result the investments are not classified in the fair value hierarchy. The fair value amounts presented in the table above are intended to permit reconciliation of the fair value hierarchy to the amounts presented in the consolidated balance sheets. The investment in the funds is subject to restrictions as detailed in Note 21(a), “Commitments and Contingent Liabilities.” (2)
Fair Value, Measured on a Recurring Basis, Gain (Loss) Included in Earnings	Twelve Months Ended December 31, 2023 Balance at beginning of year Purchases and issuances Transfers in Transfers (out) Settlements and sales Increase/(decrease) in fair value included in net income (1) / OCI (2) Balance at end of year Change in unrealized gains (losses) relating to assets held at end of year (1) (2) Assets Privately-held investments — available for sale Asset-backed securities $ — $ 14.7 $ — $ — $ — $ 0.2 $ 14.9 $ 0.2 Privately-held investments — trading Commercial mortgage loans $ 312.1 $ 40.6 $ — $ — $ (61.5) $ (16.3) $ 274.9 $ (17.9) Middle market loans and other private debt 106.9 18.3 — — (41.9) 1.5 84.8 0.5 Asset-backed securities 66.8 21.9 5.3 (5.5) (7.6) 2.0 82.9 1.7 Global corporate securities 15.0 — — — (0.4) (0.2) 14.4 (0.2) Equity securities 6.6 — — (6.6) — — — — Short-term investments 25.6 18.2 — — (25.8) — 18.0 — Total Level 3 assets $ 533.0 $ 113.7 $ 5.3 $ (12.1) $ (137.2) $ (12.8) $ 489.9 $ (15.7) Liabilities Derivative liabilities — loss portfolio transfer $ (31.7) $ — $ — $ — $ — $ 15.2 $ (16.5) $ 15.2 Total Level 3 liabilities $ (31.7) $ — $ — $ — $ — $ 15.2 $ (16.5) $ 15.2 Twelve Months Ended December 31, 2022 Assets Privately-held investments — trading Commercial mortgage loans $ 211.5 $ 215.7 $ — $ — $ (113.1) $ (2.0) $ 312.1 $ (0.5) Middle market loans and other private debt 65.3 61.8 — — (19.3) (0.9) 106.9 (2.1) Asset-backed securities 26.7 54.5 — — (12.6) (1.8) 66.8 (2.0) Global corporate securities — 15.1 — — — (0.1) 15.0 — Equity securities 3.6 5.5 — — (2.4) (0.1) 6.6 — Short-term investments — 25.6 — — — — 25.6 — Total Level 3 assets $ 307.1 $ 378.2 $ — $ — $ (147.4) $ (4.9) $ 533.0 $ (4.6) Liabilities Derivative liabilities — loss portfolio transfer $ — $ (17.2) $ — $ — $ — $ (14.5) $ (31.7) $ — Total Level 3 liabilities $ — $ (17.2) $ — $ — $ — $ (14.5) $ (31.7) $ — ______________ (1) Increases/(decreases) in the fair value of privately-held investments - trading are included in realized and unrealized investment losses in the consolidated statements of operations and other comprehensive (loss). Increases/(decreases) in the fair value of derivative liabilities - loss portfolio transfer are included within change in fair value of derivatives in the consolidated statements of operations and other comprehensive (loss). (2) Increases/(decreases) in the fair value of privately-held investments - available for sale are included in other comprehensive income (“OCI”).
Fair Value Inputs, and Valuation Techniques	At December 31, 2023 Fair Value Valuation Techniques Unobservable (U) inputs Ranges Weighted Average ($ in millions) Privately-held investments — Trading Commercial mortgage loans $ 274.9 Discounted cash flow Discount rate 3.8 % – 20.0 % 7.7 % Middle market loans and other private debt 84.8 Discounted cash flow Discount rate 7.7 % 18.5 % 11.0 % Asset-backed securities 82.9 Discounted cash flow Discount rate 5.9 % – 9.7 % 7.0 % Global corporate securities 14.4 Discounted cash flow Discount rate 6.6 % – 6.6 % 6.6 % Short-term investments 18.0 Discounted cash flow Discount rate 9.3 % 18.5 % 9.3 % Privately-held investments — Available for sale Asset-backed securities 14.9 Transaction value n/a n/a n/a n/a Total $ 489.9

Reinsurance (Tables)

Reinsurance (Tables)	12 Months Ended
	Dec. 31, 2023
Reinsurance Disclosures [Abstract]
Summary of Assumed and Ceded Reinsurance on Premiums Written, Premiums Earned and Insurance Losses and Loss Adjustment Expenses	Twelve Months Ended December 31, 2023 2022 2021 ($ in millions) Premiums written: Insurance $ 2,446.6 $ 2,531.7 $ 2,341.4 Reinsurance 1,521.0 1,807.0 1,597.0 Ceded (1,385.7) (1,442.7) (1,350.7) Net written premiums $ 2,581.9 $ 2,896.0 $ 2,587.7 Premiums earned: Insurance $ 2,444.8 $ 2,370.8 $ 2,139.1 Reinsurance 1,562.0 1,617.2 1,479.2 Ceded (1,392.3) (1,299.3) (1,207.8) Net earned premiums $ 2,614.5 $ 2,688.7 $ 2,410.5 Losses and loss adjustment expenses: Insurance $ 1,644.5 $ 1,574.2 $ 1,499.8 Reinsurance 707.2 939.5 1,000.6 Ceded (798.7) (833.7) (807.1) Losses and loss adjustment expenses $ 1,553.0 $ 1,680.0 $ 1,693.3

Derivative Contracts (Tables)

Derivative Contracts (Tables)	12 Months Ended
	Dec. 31, 2023
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Fair Value of Derivative Instruments	As at December 31, 2023 As at December 31, 2022 Derivatives Not Designated as Hedging Instruments Balance Sheet Location Notional Fair Notional Fair ($ in millions) ($ in millions) Foreign Exchange Contracts (1) Derivative assets $ 1,262.1 $ 31.4 $ 945.8 $ 41.9 Foreign Exchange Contracts Derivative liabilities $ 540.8 $ (9.3) $ 729.5 $ (3.2) Loss Portfolio Transfer Liability - Embedded Derivative (2) Derivative liabilities — $ (16.5) — (31.7) ________________ (1) Fair value is net of $3.4 million of cash collateral (December 31, 2022 — $3.7 million). (2) The LPT contains an embedded derivative within the contract in relation to the variable interest crediting rate. As at December 31, 2023 As at December 31, 2022 Derivatives Designated as Cash Flow Hedges Under ASC 815 Balance Sheet Location Notional Fair Notional Fair ($ in millions) ($ in millions) Foreign Exchange Contracts Derivative assets $ 76.9 $ 0.3 $ 109.7 $ 14.3
Gain/(Loss) Recognized in Income on Derivative	Amount of (Loss)/Gain Recognized on Derivatives For the Twelve Months Ended Location of Gain/(Loss) December 31, 2023 December 31, 2022 Derivatives not designated as hedges ($ in millions) Foreign Exchange Contracts Change in Fair Value of Derivatives 10.9 (66.0) Loss Portfolio Transfer Liability - Embedded Derivative Change in Fair Value of Derivatives 15.2 (14.5) Derivatives designated as cash flow hedges Foreign Exchange Contracts General, administrative and corporate expenses in consolidated statement of operations (8.1) 5.9 Foreign Exchange Contracts Net change from current period hedged transactions in other comprehensive income (14.0) 15.4

Deferred Policy Acquisition C_2

Deferred Policy Acquisition Costs (Tables)	12 Months Ended
	Dec. 31, 2023
Insurance [Abstract]
Reconciliation of beginning and ending deferred policy acquisition costs	Twelve Months Ended December 31, 2023 Twelve Months Ended December 31, 2022 ($ in millions) Balance at the beginning of the period $ 319.0 $ 290.8 Acquisition costs deferred 357.4 460.0 Amortization of deferred acquisition costs (380.2) (431.8) Balance at the end of the period $ 296.2 $ 319.0

Reserves for Losses and Adjus_2

Reserves for Losses and Adjustment Expenses (Tables)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Insurance [Abstract]
Schedule of Liability for Unpaid Claims and Claims Adjustment Expense		As at December 31, 2023 2022 2021 ($ in millions) Reserve for losses and LAE at the start of the year $ 7,710.9 $ 7,611.8 $ 7,165.3 Less reinsurance recoverable (4,897.7) (3,298.1) (3,195.2) Net reserve for losses and LAE at the start of the year 2,813.2 4,313.7 3,970.1 Net loss and LAE expenses disposed (1) — (1,840.1) — Movement in net reserve for losses and LAE for claims incurred: Current year 1,492.2 1,651.9 1,648.2 Prior years 60.8 28.1 45.1 Total incurred 1,553.0 1,680.0 1,693.3 Net Losses and LAE payments for claims incurred: Current year (161.1) (192.7) (729.1) Prior years (1,011.9) (1,098.4) (580.7) Total paid (1,173.0) (1,291.1) (1,309.8) Foreign exchange (gains)/losses 39.6 (49.3) (39.9) Net reserve for losses and LAE at the end of the year 3,232.8 2,813.2 4,313.7 Plus reinsurance recoverable on unpaid losses at the end of the year 4,577.8 4,897.7 3,298.1 Reserve for losses and LAE at the end of the year $ 7,810.6 $ 7,710.9 $ 7,611.8 ________________ (1) Net loss and LAE expenses disposed of $1,840.1 million represent the net loss reserves as at May 20, 2022 (“Closing Date”) for losses in relation to 2019 and prior accident years, in addition to the $770.0 million of ceded reserves under the previous ADC agreement, recognizing a total recoverable of $2,610.1 million. These reserves were rolled forward from the initial effective date of September 30, 2021, at which time the net losses reserves were $3,120.0 million.
Short-duration Insurance Contracts, Claims Development	Property Insurance Lines As at December 31, 2023 Incurred Claims, IBNR and Loss Adjustment Expenses, Net of Reinsurance Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims Accident Year For the Years Ended December 31, Unaudited Prior Years 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 $ (in millions) 2014 164.3 156.3 133.5 134.1 133.3 131.8 131.2 125.8 132.8 132.8 — 10,037 2015 237.3 203.0 197.7 200.0 200.5 197.5 181.9 193.4 193.4 — 11,624 2016 236.8 247.7 242.6 244.0 245.3 234.2 233.4 233.4 — 10,844 2017 293.8 256.8 250.0 251.3 273.0 261.6 261.6 — 9,805 2018 200.7 203.0 186.8 196.3 180.2 180.2 — 8,395 2019 125.3 129.2 102.8 109.0 109.0 — 6,944 2020 203.2 198.3 208.0 213.5 25.9 7,689 2021 207.4 202.0 195.5 16.1 6,810 2022 167.1 174.2 22.3 5,702 2023 152.1 81.7 3,297 Total $ 1,845.7 Property Insurance Lines Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Unaudited Prior Years Accident Year 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 ($ in millions) 2014 40.2 86.0 113.5 123.1 127.1 128.5 129.8 130.9 132.8 132.8 2015 56.9 141.0 168.4 177.5 193.8 192.4 193.4 193.4 193.4 2016 66.5 167.8 200.2 221.9 231.1 234.9 233.4 233.4 2017 96.0 187.3 220.0 240.9 234.8 261.6 261.6 2018 61.4 158.1 180.4 174.1 180.2 180.2 2019 48.7 90.5 98.4 109.0 109.0 2020 61.0 124.1 151.8 169.6 2021 58.7 119.7 151.2 2022 41.1 113.7 2023 30.0 Total $ 1,574.9 All outstanding liabilities for 2014 and subsequent years, net of reinsurance $ 270.8 All outstanding liabilities before 2014, net of reinsurance — Liabilities for claims and claim adjustment expenses, net of reinsurance $ 270.8 Casualty Insurance Lines As at December 31, 2023 Incurred Claims, IBNR and Loss Adjustment Expenses, Net of Reinsurance Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims Accident Year For the Years Ended December 31, Unaudited Prior Years 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 $ (in millions) 2014 142.9 125.3 137.1 127.1 134.2 138.1 135.1 141.4 107.0 107.0 — 3,962 2015 201.3 221.3 183.9 201.3 234.0 232.3 257.9 180.5 180.5 — 4,811 2016 215.0 186.0 181.3 187.8 198.9 243.5 124.3 124.3 — 4,815 2017 179.4 172.9 176.7 194.7 215.9 61.4 61.4 — 5,470 2018 121.5 124.5 134.9 164.7 43.7 43.7 — 5,455 2019 124.1 146.5 153.5 48.6 48.6 — 5,156 2020 132.9 141.6 140.4 146.6 50.2 3,753 2021 173.5 188.6 197.8 99.4 3,343 2022 204.7 215.1 148.3 3,050 2023 224.9 173.0 2,465 Total $ 1,349.9 Casualty Insurance Lines Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Unaudited Prior Years Accident Year 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 ($ in millions) 2014 2.6 13.1 32.2 58.8 71.9 95.6 108.2 106.8 107.0 107.0 2015 3.1 16.8 56.0 91.8 137.2 166.2 180.5 180.5 180.5 2016 4.1 22.5 39.6 81.5 108.4 123.3 124.3 124.3 2017 3.5 22.7 52.2 95.8 89.4 61.4 61.4 2018 3.1 27.7 42.7 58.1 43.7 43.7 2019 6.3 17.6 64.1 48.6 48.6 2020 — 9.3 36.3 61.6 2021 3.1 23.5 53.8 2022 8.90 5.6 2023 26.9 Total $ 713.4 All outstanding liabilities for 2014 and subsequent years, net of reinsurance $ 636.5 All outstanding liabilities before 2014, net of reinsurance — Liabilities for claims and claim adjustment expenses, net of reinsurance $ 636.5 Marine, Aviation and Energy Insurance Lines As at December 31, 2023 Incurred Claims, IBNR and Loss Adjustment Expenses, Net of Reinsurance Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims For the Years Ended December 31, Unaudited Prior Years Accident Year 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 $ (in millions) 2014 308.8 312.3 297.4 309.2 304.8 311.7 301.0 312.4 271.3 271.3 — 4,056 2015 295.4 297.6 280.1 284.8 308.3 311.1 318.3 267.3 267.3 — 4,067 2016 259.5 229.5 228.3 228.7 218.5 220.3 197.9 197.9 — 4,422 2017 209.6 200.2 206.7 214.2 225.6 141.4 141.4 — 6,078 2018 170.4 207.4 208.3 234.8 151.0 151.0 — 5,183 2019 145.6 153.0 123.5 102.4 102.4 — 3,691 2020 110.2 111.2 125.7 126.6 9.6 3,847 2021 93.1 96.2 95.7 10.1 4,787 2022 108.0 106.1 31.2 5,428 2023 117.4 72.6 2,703 Total $ 1,577.1 Marine, Aviation and Energy Insurance Lines Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Unaudited Prior Years Accident Year 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 ($ in millions) 2014 53.3 116.3 188.3 208.9 231.6 249.8 262.0 269.0 271.3 271.3 2015 44.7 122.7 173.3 193.0 220.9 256.5 268.4 267.3 267.3 2016 30.9 82.3 142.1 163.8 190.4 193.3 197.9 197.9 2017 40.1 97.4 140.1 168.2 149.8 141.4 141.4 2018 26.7 104.7 133.0 151.0 151.0 151.0 2019 33.5 72.5 89.6 102.4 102.4 2020 28.5 66.5 88.7 101.2 2021 23.5 52.3 64.7 2022 24.9 57.6 2023 27.8 Total $ 1,382.6 All outstanding liabilities for 2014 and subsequent years, net of reinsurance $ 194.5 All outstanding liabilities before 2014, net of reinsurance — Liabilities for claims and claim adjustment expenses, net of reinsurance $ 194.5 Financial and Professional Insurance Lines As at December 31, 2023 Incurred Claims, IBNR and Loss Adjustment Expenses, Net of Reinsurance Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims For the Years Ended December 31, Unaudited Prior Years Accident Year 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 $ (in millions) 2014 134.3 130.1 128.6 119.0 130.2 119.2 120.3 121.7 98.8 98.8 — 794 2015 173.3 174.8 184.6 188.7 189.8 184.6 196.7 145.9 145.9 — 1,083 2016 190.1 210.9 215.4 201.2 184.6 186.1 134.3 134.3 — 1,241 2017 205.5 181.6 186.5 187.1 209.1 136.0 136.0 — 1,751 2018 155.7 171.6 153.2 161.9 111.2 111.2 — 4,636 2019 248.2 261.1 238.9 132.7 132.7 — 23,826 2020 348.1 347.9 338.2 352.3 87.4 106,054 2021 286.2 304.8 296.2 128.4 34,929 2022 317.0 302.4 184.6 3,422 2023 341.6 277.0 3,137 Total $ 2,051.4 Financial and Professional Insurance Lines Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Unaudited Prior Years Accident Year 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 ($ in millions) 2014 2.9 30.5 53.2 71.8 79.2 84.9 91.7 99.3 98.8 98.8 2015 13.7 43.3 69.9 89.1 109.5 138.4 150.9 145.9 145.9 2016 15.0 71.0 101.5 129.6 125.5 130.1 134.3 134.3 2017 27.1 51.2 83.2 116.8 134.8 136.0 136.0 2018 19.1 73.4 99.0 111.2 111.2 111.2 2019 27.1 86.6 121.1 132.7 132.7 2020 47.6 121.2 174.7 226.2 2021 43.2 90.4 131.7 2022 17.8 75.5 2023 21.5 Total $ 1,213.8 All outstanding liabilities for 2014 and subsequent years, net of reinsurance $ 837.6 All outstanding liabilities before 2014, net of reinsurance — Liabilities for claims and claim adjustment expenses, net of reinsurance $ 837.6 Property Catastrophe and Other Property Reinsurance As at December 31, 2023 Incurred Claims, IBNR and Loss Adjustment Expenses, Net of Reinsurance Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims For the Years Ended December 31, Unaudited Prior Years Accident Year 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 $ (in millions) 2014 185.8 172.8 157.2 145.7 145.8 141.1 141.1 139.7 137.1 137.1 — 901 2015 211.3 185.1 175.3 154.8 170.3 170.4 176.8 155.3 155.3 — 1,051 2016 266.4 266.3 265.0 243.8 239.2 230.7 228.6 228.6 — 1,303 2017 552.3 531.4 513.1 501.8 573.6 431.8 431.8 — 1,958 2018 318.2 351.9 344.1 534.7 282.1 282.1 — 1,777 2019 228.0 241.5 332.4 162.7 162.7 — 1,401 2020 317.6 403.7 350.7 362.6 (5.5) 1,418 2021 652.9 479.3 498.9 30.7 1,498 2022 393.7 392.5 48.9 1,404 2023 230.8 117.7 753 Total $ 2,882.4 Property Catastrophe and Other Property Reinsurance Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Unaudited Prior Years Accident Year 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 ($ in millions) 2014 36.8 98.7 124.5 133.8 137.3 135.9 137.5 137.4 137.1 137.1 2015 35.8 94.2 125.4 137.8 154.8 157.1 158.6 155.3 155.3 2016 56.3 161.7 202.4 213.2 226.1 231.7 228.6 228.6 2017 123.2 356.1 414.5 438.4 411.7 431.8 431.8 2018 122.4 282.2 286.2 279.5 282.1 282.1 2019 28.1 140.8 167.5 162.7 162.7 2020 41.9 165.7 237.3 312.3 2021 75.0 235.7 364.3 2022 65.2 200.8 2023 45.1 Total $ 2,320.1 All outstanding liabilities for 2014 and subsequent years, net of reinsurance 562.3 All outstanding liabilities before 2014, net of reinsurance — Liabilities for claims and claim adjustment expenses, net of reinsurance $ 562.3 Casualty Reinsurance As at December 31, 2023 Incurred Claims, IBNR and Loss Adjustment Expenses, Net of Reinsurance Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims For the Years Ended December 31, Unaudited Prior Years Accident Year 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 $ (in millions) 2014 203.4 205.8 214.2 207.5 201.0 204.0 199.9 189.8 131.0 131.0 — 1,866 2015 192.5 199.2 208.9 211.4 209.0 205.2 195.2 116.5 116.5 — 2,067 2016 231.1 243.5 243.0 252.9 260.3 251.2 137.1 137.1 — 2,231 2017 242.7 240.4 251.0 250.4 261.6 110.8 110.8 — 2,284 2018 227.2 256.5 264.1 254.3 92.7 92.7 — 2,149 2019 233.5 254.0 244.3 52.9 52.9 — 1,790 2020 253.6 234.7 198.8 181.2 62.0 1,389 2021 206.4 216.3 205.7 93.3 1,325 2022 249.5 250.6 178.3 1,256 2023 271.6 244.9 779 Total $ 1,550.1 Casualty Reinsurance Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Unaudited Prior Years Accident Year 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 ($ in millions) 2014 2.5 13.7 37.6 59.9 86.0 106.8 124.4 131.0 131.0 131.0 2015 3.4 17.8 38.1 65.2 89.0 108.0 116.5 116.5 116.5 2016 9.1 33.3 63.6 95.6 125.6 137.1 137.1 137.1 2017 8.8 30.4 58.8 97.0 110.7 110.8 110.8 2018 7.1 33.4 73.3 92.7 92.7 92.7 2019 9.2 36.4 52.5 52.9 52.9 2020 9.1 27.8 44.3 71.9 2021 7.8 37.3 64.0 2022 9.4 31.1 2023 8.5 Total $ 816.5 All outstanding liabilities for 2014 and subsequent years, net of reinsurance 733.6 All outstanding liabilities before 2014, net of reinsurance — Liabilities for claims and claim adjustment expenses, net of reinsurance $ 733.6 Specialty Reinsurance As at December 31, 2023 Incurred Claims, IBNR and Loss Adjustment Expenses, Net of Reinsurance Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims For the Years Ended December 31, Unaudited Prior Years Accident Year 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 $ (in millions) 2014 150.7 139.0 130.9 122.1 124.9 123.5 119.3 116.4 104.0 104.0 — 618 2015 164.7 168.0 162.7 157.3 155.9 151.4 151.1 132.1 132.1 — 774 2016 237.2 238.3 236.1 228.9 224.0 211.6 188.2 188.2 — 937 2017 377.4 390.4 374.1 363.0 356.4 305.9 305.9 — 1,336 2018 393.8 393.3 391.7 416.2 324.3 324.3 — 1,418 2019 472.6 495.9 398.7 401.1 401.1 — 1,543 2020 414.1 601.3 375.4 379.5 36.7 1,494 2021 157.4 152.5 142.1 41.1 1,356 2022 194.7 194.3 108.9 1,319 2023 142.5 100.4 952 Total $ 2,314.0 Specialty Reinsurance Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Unaudited Prior Years Accident Year 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 ($ in millions) 2014 16.4 55.9 80.6 88.7 99.0 101.9 103.6 104.1 104.0 104.0 2015 17.5 55.8 103.3 120.9 129.9 132.9 132.6 132.1 132.1 2016 58.4 150.3 164.7 182.7 192.9 194.3 188.2 188.2 2017 94.5 238.2 270.0 305.2 306.4 305.9 305.9 2018 27.1 279.6 313.0 324.7 324.3 324.3 2019 273.2 381.0 399.2 401.1 401.1 2020 213.0 270.1 290.9 311.6 2021 28.3 53.4 76.1 2022 26.0 53.8 2023 22.6 Total $ 1,919.7 All outstanding liabilities for 2014 and subsequent years, net of reinsurance 394.3 All outstanding liabilities before 2014, net of reinsurance — Liabilities for claims and claim adjustment expenses, net of reinsurance $ 394.3
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability	Reconciliation of Incurred and Paid Claims Development to total Reserve for Losses and LAE As at December 31, 2023 As at December 31, 2022 ($ in millions) Net outstanding liabilities: Insurance lines - Property insurance lines 270.8 259.0 - Casualty insurance lines 636.5 457.3 - Marine, aviation and energy insurance lines 194.5 163.7 - Financial and professional insurance lines 837.6 671.9 Total insurance lines 1,939.4 1,551.9 Reinsurance lines - Property catastrophe and other property reinsurance 562.3 677.9 - Casualty reinsurance 733.6 571.6 - Specialty reinsurance 394.3 350.2 Total reinsurance lines 1,690.2 1,599.7 Net loss and LAE 3,629.6 3,151.6 Reinsurance recoverable on unpaid losses: Insurance lines 2,821.6 2,907.8 Reinsurance lines 1,756.2 1,989.9 Total reinsurance recoverable on unpaid losses 4,577.8 4,897.7 Deferred gain on retroactive contracts 27.3 42.7 Unallocated claims incurred 47.9 41.4 Other reinsurance balances recoverable (1) (489.1) (429.3) Carbon syndicate reserves 16.7 5.2 Other 0.4 1.6 (396.8) (338.4) Reserve for losses and LAE at the end of the year 7,810.6 7,710.9 ________________ (1)
Short-duration Insurance Contracts, Schedule of Historical Claims Duration		Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance (unaudited) Years 1 2 3 4 5 6 7 8 9 10 Property insurance lines 30.6 % 39.0 % 13.7 % 6.3 % 2.7 % 2.4 % 0.2 % 0.3 % 0.7 % — % Casualty insurance lines 4.1 % 18.7 % 33.1 % 24.2 % 2.7 % 1.0 % 5.1 % (0.4) % 0.1 % — % Marine, aviation and energy insurance lines 22.5 % 33.3 % 21.5 % 11.4 % 3.2 % 3.1 % 2.8 % 0.7 % 0.4 % — % Financial and professional insurance lines 12.1 % 28.6 % 20.7 % 16.0 % 5.3 % 6.0 % 4.7 % 1.4 % (0.3) % — % Property catastrophe and other property reinsurance 22.6 % 45.5 % 16.7 % 5.8 % 2.3 % 1.5 % 0.2 % (0.7) % (0.1) % — % Casualty reinsurance 6.0 % 19.0 % 22.4 % 19.3 % 12.5 % 8.1 % 5.2 % 1.7 % — % — % Specialty reinsurance 27.3 % 34.9 % 14.3 % 7.4 % 3.7 % 1.1 % (0.5) % — % — % — %

Income Taxes (Tables)

Income Taxes (Tables)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Income Tax Disclosure [Abstract]
Summary of Total Income Tax	Twelve Months Ended December 31, 2023 2022 2021 ($ in millions) Income tax (benefit)/expense allocated to net income $ (132.1) $ (78.1) $ 5.3 Income tax expense/(benefit) allocated to other comprehensive income 20.6 (23.9) 0.3 Total income tax (benefit)/expense $ (111.5) $ (102.0) $ 5.6
Schedule of Income Tax by Taxing Authority	Twelve Months Ended December 31, 2023 Income Current tax Deferred tax Total income tax ($ in millions) Bermuda (1) $ 148.8 $ — $ (201.1) $ (201.1) U.S. (2) 236.3 52.4 3.0 55.4 U.K. (3) 0.7 5.3 0.1 5.4 Other (4) 16.8 7.9 0.3 8.2 Total $ 402.6 $ 65.6 $ (197.7) $ (132.1) Twelve Months Ended December 31, 2022 (Loss)/income Current tax Deferred tax Total income tax ($ in millions) Bermuda $ (103.3) $ — $ — $ — U.S. 34.8 14.8 (102.9) (88.1) U.K. 62.4 7.0 — 7.0 Other (20.9) 4.7 (1.7) 3.0 Total $ (27.0) $ 26.5 $ (104.6) $ (78.1) Twelve Months Ended December 31, 2021 Income/(loss) Current tax Deferred tax Total income tax ($ in millions) Bermuda $ 22.9 $ — $ — $ — U.S 5.0 5.8 — 5.8 U.K. 75.1 — (0.3) (0.3) Other (67.9) 2.7 (2.9) (0.2) Total $ 35.1 $ 8.5 $ (3.2) $ 5.3
Income Tax Reconciliation		Twelve Months Ended December 31, 2023 2022 2021 Income Tax Reconciliation ($ in millions) Income tax benefit at statutory tax rate of zero percent $ — $ — $ — Overseas statutory tax rates differential 56.3 16.8 (0.9) Base erosion and anti-abuse tax (BEAT) expense 0.9 2.3 6.1 Prior year adjustments (1) 6.9 (2.9) 0.5 Introduction of Bermuda corporate income tax (201.1) — — Change in valuation allowance (2) 4.0 (98.9) 9.6 Impact of unrecognized tax benefits (3) — — — Australian non-resident withholding tax — 1.5 0.6 Foreign exchange (1.3) (0.3) (1.5) Non-deductible expenses 2.5 2.4 2.4 Impact of changes in statutory tax rates (0.3) (5.7) (11.5) Tax effect of OCI in income statement — 6.7 — Total income tax (benefit)/expense $ (132.1) $ (78.1) $ 5.3 ________________ (1) The submission dates for filing income tax returns for the Company’s U.S. and U.K. operating subsidiaries are after the submission date of this report. Accordingly, the final tax liabilities may differ from the estimated income tax expense included in this report and may result in prior year adjustments being reported. The prior period adjustments for the twelve months ended December 31, 2023 predominantly relate to the determination of the results of the branches of the U.K. operating subsidiaries. The prior period adjustments for the twelve months ended December 31, 2022 and 2021 predominantly relate to the determination of results in the U.K. (2) The decrease in valuation allowance in 2022 related to a change in judgment about the recoverability of deferred tax assets in the U.S. operating subsidiaries. (3) In 2023, the Company did not have any unrecognized tax benefits.
Tax Effects of Deferred Tax Assets and Deferred Tax Liabilities	As at December 31, 2023 2022 ($ in millions) D eferred tax assets: Operating loss carryforwards 217.2 167.6 Capital loss carryforwards 9.7 6.7 Insurance reserves: Losses and loss adjustment expenses 104.1 28.3 Unrealized losses on investments 8.9 20.6 Accrued expenses 13.4 6.1 Foreign tax credit carryforwards 19.0 19.8 Insurance reserves: Unearned premiums 35.0 36.0 Intangible assets 82.9 0.7 Office properties and equipment 16.5 14.2 Operating lease liabilities 15.6 18.5 Other temporary differences 7.6 3.7 Total deferred tax assets 529.9 322.2 Less valuation allowance (172.7) (145.7) Deferred tax assets, net of valuation allowance $ 357.2 $ 176.5 Deferred tax liabilities: Deferred acquisition costs (32.4) (37.0) Right-of-use operating lease assets (10.4) (13.7) Insurance reserves: Losses and loss adjustment expenses (0.1) (0.2) Other temporary differences (3.3) (6.4) Total deferred tax (liabilities) (46.2) (57.3) Net deferred tax assets $ 311.0 $ 119.2

Capital Structure (Tables)

Capital Structure (Tables)	12 Months Ended
	Dec. 31, 2023
Equity [Abstract]
Summary of Authorized and Issued Share Capital	As at December 31, 2023 At December 31, 2022 Number $ in Number $ in Authorized share capital: Ordinary Shares $0.01 per share (2022 — $0.01 per share) 70,000,000 700 70,000,000 700 Preference Shares 0.15144558¢ per share 30,000,000 45 30,000,000 45 Total authorized share capital 745 745 Issued share capital: Issued ordinary shares $0.01 per share (2022 — $0.01 per share) 60,395,839 604 60,395,839 604 Issued 5.950% preference shares of 0.15144558¢ each with a liquidation preference of $25 per share 11,000,000 17 11,000,000 17 Issued 5.625% preference shares of 0.15144558¢ each with a liquidation preference of $25 per share 10,000,000 15 10,000,000 15 Issued 5.625% preference shares of 0.15144558¢ represented by depositary shares, each with a liquidation preference of $25 per share (1) 10,000 — 10,000 — Total issued share capital 636 636 ________________ (1) Each depositary share represents a 1/1000th interest in a share of the 5.625% preference shares.

Earnings Per Share (Tables)

Earnings Per Share (Tables)	12 Months Ended
	Dec. 31, 2023
Earnings Per Share [Abstract]
Schedule of Earnings Per Share, Basic and Diluted	Twelve Months Ended December 31, 2023 2022 2021 ($ in millions, except share and share amounts) Net income $ 534.7 $ 51.1 $ 29.8 Less: Preference share dividends (49.9) (44.6) (44.5) Net income/(loss) available to ordinary shareholders $ 484.8 $ 6.5 $ (14.7) Basic and diluted weighted average ordinary shares outstanding 60,395,839 60,395,839 60,395,839 Basic and diluted earnings/(loss) per ordinary share $ 8.03 $ 0.11 $ (0.24)

Statutory Requirements and Di_2

Statutory Requirements and Dividends Restrictions (Tables)	12 Months Ended
	Dec. 31, 2023
Disclosure Statutory Requirements And Dividends Restrictions Summary Of Statutory Requirements And Dividends Restrictions [Abstract]
Summary of Statutory Capital and Surplus Requirements	As at December 31, 2023 U.S. Bermuda U.K. ($ in millions) Required statutory capital and surplus $ 488.9 $ 601.1 $ 257.2 Actual statutory capital and surplus $ 1,063.1 $ 1,685.2 $ 734.7 As at December 31, 2022 U.S. Bermuda U.K. ($ in millions) Required statutory capital and surplus $ 513.9 $ 536.7 $ 771.8 Actual statutory capital and surplus $ 838.6 $ 1,426.6 $ 802.5

Dividends (Tables)

Dividends (Tables)	12 Months Ended
	Dec. 31, 2023
Dividends [Abstract]
Summary of Declared Dividends	In the twelve months ended December 31, 2023, the Company’s Board of Directors paid the following dividends: Calendar Quarter Preference Share Category Quarterly Total Declared Paid 5.950% PS 5.625% PS 5.625% DS Q1 2023 $ 4,090,900 $ 3,516,000 $ 3,515,600 $ 11,122,500 03/01/23 04/01/23 Q2 2023 $ 4,090,900 $ 3,516,000 $ 3,515,600 $ 11,122,500 06/01/23 07/01/23 Q3 2023 $ 6,815,600 $ 3,516,000 $ 3,515,600 $ 13,847,200 09/01/23 10/01/23 Q4 2023 $ 6,741,900 $ 3,516,000 $ 3,515,600 $ 13,773,500 11/30/23 12/28/23 Total Paid $ 21,739,300 $ 14,064,000 $ 14,062,400 $ 49,865,700 ______________ (1) 5.950% Preference Shares (AHL PRC) — Fixed to Floating Rate Perpetual Non-Cumulative Preference Shares 5.625% Preference Shares (AHL PRD) — Perpetual Non-Cumulative Preference Shares 5.625% Preference Shares(AHL PRE) are represented by depositary shares, each representing a 1/1000 th interest in a share of the 5.625% Preference Shares. The dividend paid per depositary share is likewise 1/1000 th of the declared dividend, equivalent to $0.35156 per depositary share. In the twelve months ended December 31, 2023, the Company paid an ordinary shares dividend of $40.3 million to Highlands Bermuda Holdco, Ltd., the holder of all the Company’s ordinary shares.

Operating Leases (Tables)

Operating Leases (Tables)	12 Months Ended
	Dec. 31, 2023
Leases [Abstract]
Lease, Cost	Operating lease charge. The following table summarizes the operating lease charge for the twelve months ended December 31, 2023, 2022 and 2021: For the Twelve Months Ended December 31, 2023 December 31, 2022 December 31, 2021 ($ in millions) Amortization charge on right-of-use operating leased assets $ 10.7 $ 10.1 $ 12.0 Interest on operating lease liabilities 4.5 5.4 5.5 Operating lease charge $ 15.2 $ 15.5 $ 17.5
Lessee, Operating Lease, Liability	Lease Liabilities. The following table summarizes the maturity of lease liabilities under non-cancellable leases as of December 31, 2023 and 2022: December 31, 2023 December 31, 2022 ($ in millions) Operating leases — maturities 2023 $ — $ 15.4 2024 15.4 15.2 2025 15.1 14.9 2026 14.3 14.1 2027 12.8 12.6 2028 12.7 12.4 Later years 32.4 34.8 Total minimum lease payments $ 102.7 $ 119.4 Less imputed interest (16.6) (23.9) Total lease liabilities $ 86.1 $ 95.5
Summary of Other Lease Information	Other lease information. The following table summarizes the cash flows on operating leases for the twelve months ended December 31, 2023, 2022 and 2021 and other supplemental information: For the Twelve Months Ended December 31, 2023 December 31, 2022 December 31, 2021 ($ in millions) Cash paid for amounts included in the measurement of lease liabilities - Operating cash outflow from operating leases $ (15.5) $ (15.5) $ (17.5) Right-of-use assets obtained in exchange for lease obligations - Operating leases $ 0.2 $ 1.9 $ 23.9 Reduction to Right-of-use assets resulting from reductions to lease obligations - Operating leases $ 0.1 $ 7.0 $ 2.3 Weighted Averages - Operating leases, remaining lease terms (years) 7.3 8.1 8.8 - Operating leases, average discount rate 5.0 % 5.0 % 5 %

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2023
Commitments and Contingencies Disclosure [Abstract]
Schedule of Company's Restricted Assets	The following table details the forms and value of Company’s material restricted assets as at December 31, 2023 and 2022: As at December 31, 2023 At December 31, 2022 ($ in millions, except percentages) Regulatory trusts and deposits: Affiliated transactions $ 660.8 $ 707.1 Third party 2,714.4 2,817.7 Letters of credit / guarantees (1) 172.0 471.3 Total restricted assets (excluding illiquid assets) 3,547.2 3,996.1 Other investments — illiquid assets 209.3 221.3 Total restricted assets and illiquid assets $ 3,756.5 $ 4,217.4 Total as percent of cash and invested assets (2) 50.2 % 59.4 %

Reclassifications from Accumu_2

Reclassifications from Accumulated Other Comprehensive Income (Tables)	12 Months Ended
	Dec. 31, 2023
Equity [Abstract]
Schedule of Accumulated Other Comprehensive Income Reclassification	The following table sets out the components of the Company’s AOCI that are reclassified into the condensed consolidated statement of operations for the twelve months ended December 31, 2023, 2022 and 2021: Amount Reclassified from AOCI Details about the AOCI Components Twelve Months Ended December 31, 2023 Twelve Months Ended December 31, 2022 Twelve Months Ended December 31, 2021 Affected Line Item in the ($ in millions) Available for sale: Realized (gains) on sale of securities $ (2.2) $ (3.9) $ (24.8) Realized and unrealized investment gains Realized losses on sale of securities 42.4 58.9 4.4 Realized and unrealized investment losses 40.2 55.0 (20.4) Income from operations before income tax Tax on net realized gains of securities (6.6) — — Income tax (expense)/benefit $ 33.6 $ 55.0 $ (20.4) Net income Realized derivatives: Net realized gains on settled derivatives (8.1) 15.4 (6.2) General, administrative and corporate expenses Tax on settled derivatives — — — Income tax (expense)/benefit $ (8.1) $ 15.4 $ (6.2) Net income Total reclassifications from AOCI to the statement of operations, net of income tax $ 25.5 $ 70.4 $ (26.6) Net income

Credit Facility and Long-term D

Credit Facility and Long-term Debt (Tables)	12 Months Ended
	Dec. 31, 2023
Debt Disclosure [Abstract]
Summary of Contractual Obligations Under Short-term Debts	The following table summarizes our contractual obligations under long-term debt as at December 31, 2023. Payments Due By Period Contractual Basis Less than 1 year 1-3 years 3-5 years More than 5 years Total ($ in millions) Long-term Debt Obligations $ — $ 300.0 $ — $ — $ 300.0

Credit Losses (Tables)

Credit Losses (Tables)	12 Months Ended
	Dec. 31, 2023
Credit Loss [Abstract]
Debt Securities, Available-for-Sale, Allowance for Credit Loss	The following tables summarize the Company’s allowance for expected credit losses for the twelve months ended December 31, 2023 and December 31, 2022 in available for sale investments, reinsurance recoverables and receivables: Available for Sale Investments December 31, 2023 2022 ($ in millions) Balance at the beginning of the period $ 7.7 $ 2.7 Additions to the allowance for credit losses on securities for which credit losses were not previously recognized 0.3 5.4 Increases/(decreases) to the allowance for credit losses on securities that had an allowance in the prior period (3.6) 0.2 Reductions to the allowance for securities sold (1.5) (0.6) Balance at the end of the period $ 2.9 $ 7.7
Reinsurance Recoverable, Allowance for Credit Loss	December 31, 2023 December 31, 2022 ($ in millions) ($ in millions) Reinsurance Recoverables Receivables Reinsurance Recoverables Receivables Balance at the beginning of the year $ 3.7 $ 25.0 $ 3.3 $ 30.2 Movement in the year — (4.0) 0.4 (5.2) Balance at the end of the year $ 3.7 $ 21.0 $ 3.7 $ 25.0

Basis of Preparation and Sign_3

Basis of Preparation and Significant Accounting Policies - Additional Information (Details)	12 Months Ended
	Dec. 31, 2023 period
Property, Plant and Equipment [Line Items]
Non-Payment Of Dividends, Number Of Periods	6
Computer Equipment | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	3 years
Computer Equipment | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	5 years
Furniture and Fixtures
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	4 years
Leasehold Improvements
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	15 years
Computer Software, Intangible Asset | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	3 years
Computer Software, Intangible Asset | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	5 years

Segment Reporting - Additional

Segment Reporting - Additional Information - Narrative (Details)	12 Months Ended
	Dec. 31, 2023 segement
Segment Reporting [Abstract]
Number of business segments	2

Segment Reporting - Summary of

Segment Reporting - Summary of Gross and Net Written and Earned Premiums, Underwriting Results, Ratios and Reserves for Each of Company's Business Segments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Segment Reporting Information [Line Items]
Gross written premiums	$ 3,967.6	$ 4,338.7	$ 3,938.4
Net written premiums	2,581.9	2,896	2,587.7
Gross earned premiums	4,006.8	3,988	3,618.3
Net earned premiums	2,614.5	2,688.7	2,410.5
Losses and loss adjustment expenses	1,553	1,680	1,693.3
Acquisition costs	380.2	431.8	414.1
General and administrative expenses	354.5	386.5	333.1
Underwriting income (loss)	326.8	190.4	(30)
Corporate Expenses	(114)	(71.7)	(64.3)
Non-operating expenses (1)	(35.1)	(36)	(20.6)
Net investment income (1)	275.7	188.1	147.5
Realized and unrealized investment gains	75.9	5	56.2
Realized and unrealized investment losses	(61.4)	(182.6)	(47.4)
Change in fair value of derivatives	26.1	(80.5)	(35.9)
Interest Expense, Debt	(55.2)	(43.7)	(14.3)
Net realized and unrealized foreign exchange gains	(36.2)	15.9	40
Other income	0	8.2	14.7
Other expenses	0	20.1	10.8
Income (loss) from operations before income taxes	402.6	(27)	35.1
Income Tax Expense (Benefit)	132.1	78.1	(5.3)
Net income/(loss)	534.7	51.1	29.8
Losses and loss adjustment expenses	7,810.6	7,710.9	7,611.8	$ 7,165.3
Liability for Unpaid Claims and Claims Adjustment Expense, Net, Total	$ 3,232.8	$ 2,813.2	$ 4,313.7	$ 3,970.1
Ratios
Loss ratio	59.40%	62.50%	70.20%
Acquisition cost ratio	14.50%	16.10%	17.20%
General and administrative expense ratio	13.60%	14.40%	13.80%
Expense ratio	28.10%	30.50%	31%
Combined ratio	87.50%	93%	101.20%
Reinsurance
Segment Reporting Information [Line Items]
Net written premiums	$ 1,098	$ 1,426.4	$ 1,199
Net earned premiums	1,154.5	1,251.8	1,118.8
Losses and loss adjustment expenses	1,373.1	1,360.7	2,148.4
Liability for Unpaid Claims and Claims Adjustment Expense, Net, Total	$ 1,373.1	$ 1,360.7	$ 2,148.4
Ratios
Loss ratio	52.90%	61.50%	63%
Acquisition cost ratio	18.10%	20.20%	19.80%
General and administrative expense ratio	10.40%	11.40%	10.80%
Expense ratio	28.50%	31.60%	30.60%
Combined ratio	81.40%	93.10%	93.60%
Insurance
Segment Reporting Information [Line Items]
Net written premiums	$ 1,483.9	$ 1,469.6	$ 1,388.7
Net earned premiums	1,460	1,436.9	1,291.7
Losses and loss adjustment expenses	1,859.7	1,452.5	2,165.3
Liability for Unpaid Claims and Claims Adjustment Expense, Net, Total	$ 1,859.7	$ 1,452.5	$ 2,165.3
Ratios
Loss ratio	64.50%	63.30%	76.50%
Acquisition cost ratio	11.80%	12.50%	14.90%
General and administrative expense ratio	16%	17%	16.40%
Expense ratio	27.80%	29.50%	31.30%
Combined ratio	92.30%	92.80%	107.80%
Operating Segments | Reinsurance
Segment Reporting Information [Line Items]
Gross written premiums	$ 1,521	$ 1,807	$ 1,597
Net written premiums	1,098	1,426.4	1,199
Gross earned premiums	1,562	1,617.2	1,479.2
Net earned premiums	1,154.5	1,251.8	1,118.8
Losses and loss adjustment expenses	611.1	770.3	705.2
Acquisition costs	208.6	252.4	221.6
General and administrative expenses	120.6	142.5	121.3
Underwriting income (loss)	214.2	86.6	70.7
Operating Segments | Insurance
Segment Reporting Information [Line Items]
Gross written premiums	2,446.6	2,531.7	2,341.4
Net written premiums	1,483.9	1,469.6	1,388.7
Gross earned premiums	2,444.8	2,370.8	2,139.1
Net earned premiums	1,460	1,436.9	1,291.7
Losses and loss adjustment expenses	941.9	909.7	988.1
Acquisition costs	171.6	179.4	192.5
General and administrative expenses	233.9	244	211.8
Underwriting income (loss)	$ 112.6	$ 103.8	$ (100.7)

Segment Reporting - Summary o_2

Segment Reporting - Summary of the Schedule of the Company's gross written premiums based on the location of the insured risk (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Gross written premiums	$ 3,967.6	$ 4,338.7	$ 3,938.4
Australia/Asia
Segment Reporting Information [Line Items]
Gross written premiums	177.8	257.5	275.8
Europe
Segment Reporting Information [Line Items]
Gross written premiums	179.4	194.5	140.6
United Kingdom
Segment Reporting Information [Line Items]
Gross written premiums	532.5	485.8	393.2
United States & Canada
Segment Reporting Information [Line Items]
Gross written premiums	2,472	2,715.7	2,301.8
Worldwide excluding United States
Segment Reporting Information [Line Items]
Gross written premiums	28.8	24.2	31.5
Worldwide including United States
Segment Reporting Information [Line Items]
Gross written premiums	417.2	541.7	592.2
Other (4)
Segment Reporting Information [Line Items]
Gross written premiums	$ 159.9	$ 119.3	$ 203.3

Investments - Summary of Invest

Investments - Summary of Investment Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	$ (287.3)	$ (198.4)	$ (159.2)
Investment expenses	(11.6)	(10.3)	(11.7)
Net investment income	275.7	188.1	147.5
Fixed income securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	(115.7)	(95.6)	(87.2)
Short-term investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	(5.3)	(0.6)	(0.1)
Short-term investment, Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	(0.3)	(0.1)	0
Fixed term deposits (included in cash and cash equivalents)
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	(39.9)	(6.6)	(0.7)
Catastrophe bonds — Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	(0.2)	(0.3)	(0.9)
Privately-held investments, Available for Sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	(0.1)	0	0
Privately-held investments — Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	(44.7)	(24.3)	(18.2)
Other investments, at fair value
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	(17.8)	(13.9)	(21.9)
Fixed Income Securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	$ (98.9)	$ (57)	$ (30.2)

Investments - Net Realized and

Investments - Net Realized and Unrealized Investment Gains and Losses and Change in Unrealized Gains and Losses on Investments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income securities - gross realized gains	$ 1.6	$ 2.9	$ 22.7
Fixed income securities - gross realizes (losses)	(41.5)	(58.4)	(3.6)
Net change in expected credit gains (losses)	4.8	(5)	(2.5)
Fixed income securities, trading - gross realized gains	1	0.2	12.2
Fixed income securities, trading, gross realizes (losses)	(3.5)	(1.8)	(2)
Privately-held investments net unrealized gains/(losses)	65.9	(113.9)	(23.4)
Catastrophe bonds	0.1	0.2	(0.8)
Total net realized and unrealized investment (losses)/gains recorded in the consolidated statement of operations	14.5	(177.6)	8.8
Change in available for sale net unrealized (losses)/gains:
Fixed income securities	126.2	(391.7)	(157.6)
Income tax benefit/(expense)	(20.6)	23.9	(0.3)
Total change in net unrealized (losses)/gains, net of taxes recorded in other comprehensive income	105.6	(367.8)	(157.9)
Privately-held investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income securities, trading - gross realized gains	0.8	0.7	0.6
Fixed income securities, trading, gross realizes (losses)	0	(0.1)	(13.8)
Privately-held investments net unrealized gains/(losses)	15.2	2.5	(18.1)
Short-term investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income securities - gross realized gains	0.6	1	2
Fixed income securities - gross realizes (losses)	(0.9)	(0.5)	(0.8)
Fixed income securities, trading - gross realized gains	0.1	0	0.1
Fixed income securities, trading, gross realizes (losses)	(0.3)	0	(0.3)
MVI
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Income/(loss) from equity method investment	0.2	0	0.1
Multi-Line Insurer
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Income/(loss) from equity method investment	$ 0.8	$ (0.4)	$ 0.2

Investments - Cost, Gross Unrea

Investments - Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Available for Sale Investments in Fixed Income Maturities and Short-Term Investments (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Schedule of Trading Securities and Other Trading Items [Line Items]
Fixed income securities, available for sale amortized cost	$ 4,330	$ 4,131.3
Gross unrealized gains	17.3	3.6
Gross unrealized losses	(221.8)	(338.6)
Debt Securities, Available-for-Sale, Allowance for Credit Loss	(2.9)	(7.7)
Fair market value	4,122.6	3,788.6
Total short-term investments — Available for sale
Schedule of Trading Securities and Other Trading Items [Line Items]
Gross unrealized gains	0	0
Gross unrealized losses	0	(0.4)
Debt Securities, Available-for-Sale, Allowance for Credit Loss	0	0
Fixed maturities
Schedule of Trading Securities and Other Trading Items [Line Items]
Fixed income securities, available for sale amortized cost	4,438.3	4,183.7
Gross unrealized gains	17.5	3.6
Gross unrealized losses	(221.8)	(339)
Debt Securities, Available-for-Sale, Allowance for Credit Loss	(2.9)	(7.7)
Fair market value	4,231.1	3,840.6
U.S. government
Schedule of Trading Securities and Other Trading Items [Line Items]
Fixed income securities, available for sale amortized cost	1,224.9	1,003.7
Gross unrealized gains	4.4	0
Gross unrealized losses	(26.7)	(50.7)
Debt Securities, Available-for-Sale, Allowance for Credit Loss	0	0
Fair market value	1,202.6	953
U.S. agency
Schedule of Trading Securities and Other Trading Items [Line Items]
Fixed income securities, available for sale amortized cost	7.5	9.2
Gross unrealized gains	0	0
Gross unrealized losses	(0.3)	(0.4)
Debt Securities, Available-for-Sale, Allowance for Credit Loss	0	0
Fair market value	7.2	8.8
Municipal
Schedule of Trading Securities and Other Trading Items [Line Items]
Fixed income securities, available for sale amortized cost	133.6	159.9
Gross unrealized gains	0	0
Gross unrealized losses	(5)	(9.4)
Debt Securities, Available-for-Sale, Allowance for Credit Loss	(0.5)	(1)
Fair market value	128.1	149.5
Corporate
Schedule of Trading Securities and Other Trading Items [Line Items]
Fixed income securities, available for sale amortized cost	2,051.1	2,016.9
Gross unrealized gains	12.1	3.5
Gross unrealized losses	(101.5)	(169.1)
Debt Securities, Available-for-Sale, Allowance for Credit Loss	(2.4)	(6.3)
Fair market value	1,959.3	1,845
Non-U.S. government-backed corporate
Schedule of Trading Securities and Other Trading Items [Line Items]
Fixed income securities, available for sale amortized cost	106.5	119.4
Gross unrealized gains	0.1	0
Gross unrealized losses	(5.9)	(8.8)
Debt Securities, Available-for-Sale, Allowance for Credit Loss	0	(0.2)
Fair market value	100.7	110.4
Non-U.S government
Schedule of Trading Securities and Other Trading Items [Line Items]
Fixed income securities, available for sale amortized cost	279.9	225.2
Gross unrealized gains	0.6	0.1
Gross unrealized losses	(6.7)	(11.5)
Debt Securities, Available-for-Sale, Allowance for Credit Loss	0	(0.2)
Fair market value	273.8	213.6
Agency commercial mortgage-backed securities
Schedule of Trading Securities and Other Trading Items [Line Items]
Fixed income securities, available for sale amortized cost	6.6	6.6
Gross unrealized gains	0	0
Gross unrealized losses	(0.8)	(1)
Debt Securities, Available-for-Sale, Allowance for Credit Loss	0	0
Fair market value	5.8	5.6
Agency mortgage-backed
Schedule of Trading Securities and Other Trading Items [Line Items]
Fixed income securities, available for sale amortized cost	519.9	590.4
Gross unrealized gains		0
Gross unrealized losses		(87.7)
Debt Securities, Available-for-Sale, Allowance for Credit Loss		0
Fair market value	445.1	502.7
Residential Mortgage-Backed Securities
Schedule of Trading Securities and Other Trading Items [Line Items]
Fixed income securities, available for sale amortized cost	519.9
Gross unrealized gains	0.1
Gross unrealized losses	(74.9)
Debt Securities, Available-for-Sale, Allowance for Credit Loss	0
Fair market value	445.1
Total short-term investments — Available for sale
Schedule of Trading Securities and Other Trading Items [Line Items]
Fixed income securities, available for sale amortized cost	93.6	52.4
Fair market value	93.6	$ 52
Privately-held investments, Available for Sale
Schedule of Trading Securities and Other Trading Items [Line Items]
Fixed income securities, available for sale amortized cost	14.7
Gross unrealized gains	0.2
Gross unrealized losses	0
Debt Securities, Available-for-Sale, Allowance for Credit Loss	0
Fair market value	$ 14.9

Investments - Cost, Gross Unr_2

Investments - Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Trading Investments in Fixed Income Maturities (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Schedule of Trading Securities and Other Trading Items [Line Items]
Debt Securities, Trading, Amortized Cost	$ 1,527	$ 1,576.7
High yield loans
Schedule of Trading Securities and Other Trading Items [Line Items]
Debt Securities, Trading, Amortized Cost	90.8	90.2
Gross Unrealized Gains	1.3	0.2
Gross Unrealized Losses	0	(2.1)
Fair market value, trading	92.1	88.3
Fixed income securities - trading
Schedule of Trading Securities and Other Trading Items [Line Items]
Debt Securities, Trading, Amortized Cost	1,527	1,576.7
Gross Unrealized Gains	4.7	0.7
Gross Unrealized Losses	(46)	(101.9)
Fair market value, trading	1,485.7	1,475.5
Short-term investments
Schedule of Trading Securities and Other Trading Items [Line Items]
Debt Securities, Trading, Amortized Cost	2.1	6.3
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair market value, trading	2.1	6.3
Catastrophe Bonds
Schedule of Trading Securities and Other Trading Items [Line Items]
Debt Securities, Trading, Amortized Cost	1.6	5.1
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(2.2)
Fair market value, trading	1.6	2.9
Commercial mortgage loans
Schedule of Trading Securities and Other Trading Items [Line Items]
Debt Securities, Trading, Amortized Cost	293.2	312.6
Gross Unrealized Gains	1	0.6
Gross Unrealized Losses	(19.3)	(1.1)
Fair market value, trading	274.9	312.1
Middle market loans
Schedule of Trading Securities and Other Trading Items [Line Items]
Debt Securities, Trading, Amortized Cost	85.9	109
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1.1)	(2.1)
Fair market value, trading	84.8	106.9
Securities, asset-backed
Schedule of Trading Securities and Other Trading Items [Line Items]
Debt Securities, Trading, Amortized Cost	83.1	68.8
Gross Unrealized Gains	0.4	0
Gross Unrealized Losses	(0.6)	(2)
Fair market value, trading	82.9	66.8
Global corporates securities
Schedule of Trading Securities and Other Trading Items [Line Items]
Debt Securities, Trading, Amortized Cost	14.7	15.1
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(0.3)	(0.1)
Fair market value, trading	14.4	15
Equity securities — Trading
Schedule of Trading Securities and Other Trading Items [Line Items]
Debt Securities, Trading, Amortized Cost		6.6
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Fair market value, trading		6.6
Short-term investments
Schedule of Trading Securities and Other Trading Items [Line Items]
Debt Securities, Trading, Amortized Cost	18	25.6
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair market value, trading	18	25.6
Privately-held investments
Schedule of Trading Securities and Other Trading Items [Line Items]
Debt Securities, Trading, Amortized Cost	494.9	537.7
Gross Unrealized Gains	1.4	0.6
Gross Unrealized Losses	(21.3)	(5.3)
Fair market value, trading	475	533
Total - Trading
Schedule of Trading Securities and Other Trading Items [Line Items]
Debt Securities, Trading, Amortized Cost	2,025.6	2,125.8
Gross Unrealized Gains	6.1	1.3
Gross Unrealized Losses	(67.3)	(109.4)
Fair market value, trading	1,964.4	2,017.7
U.S. government
Schedule of Trading Securities and Other Trading Items [Line Items]
Debt Securities, Trading, Amortized Cost	248.7	267.9
Gross Unrealized Gains	0.5	0
Gross Unrealized Losses	(3.7)	(6.3)
Fair market value, trading	245.5	261.6
Municipal
Schedule of Trading Securities and Other Trading Items [Line Items]
Debt Securities, Trading, Amortized Cost	3.3	3.9
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(0.2)	(0.3)
Fair market value, trading	3.1	3.6
Corporate
Schedule of Trading Securities and Other Trading Items [Line Items]
Debt Securities, Trading, Amortized Cost	178.8	175.3
Gross Unrealized Gains	0.7	0.3
Gross Unrealized Losses	(8)	(13.5)
Fair market value, trading	171.5	162.1
Non-U.S government
Schedule of Trading Securities and Other Trading Items [Line Items]
Debt Securities, Trading, Amortized Cost	35.8	32.2
Gross Unrealized Gains	0.1	0
Gross Unrealized Losses	(1.1)	(1.8)
Fair market value, trading	34.8	30.4
Asset-backed
Schedule of Trading Securities and Other Trading Items [Line Items]
Debt Securities, Trading, Amortized Cost	936	970.3
Gross Unrealized Gains	2.1	0.2
Gross Unrealized Losses	(29.9)	(74)
Fair market value, trading	908.2	896.5
Agency mortgage-backed
Schedule of Trading Securities and Other Trading Items [Line Items]
Debt Securities, Trading, Amortized Cost	25	24.7
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(2.8)	(3.3)
Fair market value, trading	22.2	21.4
Non-U.S. government-backed corporate
Schedule of Trading Securities and Other Trading Items [Line Items]
Debt Securities, Trading, Amortized Cost	8.6	12.2
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(0.3)	(0.6)
Fair market value, trading	$ 8.3	$ 11.6

Investments - Other Investments

Investments - Other Investments (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2023	Dec. 31, 2022
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment, beginning balance		$ 6.2	$ 3.9
Investment in the period		0.4	2
Proceeds from Sale of Equity Method Investments		0	0
Closing value of investment, ending balance		7.6	6.2
Fair Value, Recurring [Member]
Equity Method Investment, Aggregate Cost [Roll Forward]
Investments, fair value		117.1	134.6
Equity Method Investment, Nonconsolidated Investee, Other
Equity Method Investment, Aggregate Cost [Roll Forward]
Unrealized Gain (Loss) on Investments		1	0.3
MVI
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment, beginning balance		0.8	0.5
Investment in the period	$ 0.8	0	0.4
Proceeds from Sale of Equity Method Investments		0	0
Closing value of investment, ending balance		1	0.8
MVI | Equity Method Investment, Nonconsolidated Investee, Other
Equity Method Investment, Aggregate Cost [Roll Forward]
Unrealized Gain (Loss) on Investments		0.2	(0.1)
Multi-Line Insurer
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment, beginning balance		5.2	3.2
Investment in the period		0.4	1.6
Proceeds from Sale of Equity Method Investments		0	0
Closing value of investment, ending balance		6.4	5.2
Multi-Line Insurer | Equity Method Investment, Nonconsolidated Investee, Other
Equity Method Investment, Aggregate Cost [Roll Forward]
Unrealized Gain (Loss) on Investments		0.8	0.4
Digital Re
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment, beginning balance		0.2	0.2
Investment in the period		0	0
Proceeds from Sale of Equity Method Investments		0	0
Closing value of investment, ending balance		0.2	0.2
Digital Re | Equity Method Investment, Nonconsolidated Investee, Other
Equity Method Investment, Aggregate Cost [Roll Forward]
Unrealized Gain (Loss) on Investments		$ 0	$ 0

Investments - Summary of Fixed

Investments - Summary of Fixed Maturities (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Fixed income securities, available for sale amortized cost	$ 4,330	$ 4,131.3
Available for sale investments in fixed income maturities, Fair Market Value	4,122.6	3,788.6
Total fixed income securities — Available for sale
Debt Securities, Available-for-sale [Line Items]
Due one year or less, Cost or Amortized Cost	512	231.5
Due after one year through five years, Cost or Amortized Cost	2,889	2,526.9
Due after five years through ten years, Cost or Amortized Cost	495.1	815.1
Due after ten years, Cost or Amortized Cost	15.7	13.2
Fixed income securities, available for sale amortized cost	3,911.8	3,586.7
Due one year or less, Fair Market Value	505.8	228
Due after one year through five years, Fair Market Value	2,818.9	2,385.7
Due after five years through ten years, Fair Market Value	440.3	707.7
Due after ten years, Fair Market Value	15.2	10.9
Available for sale investments in fixed income maturities, Fair Market Value	3,780.2	3,332.3
Agency mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Fixed income securities, available for sale amortized cost	519.9	590.4
Available for sale investments in fixed income maturities, Fair Market Value	445.1	502.7
Agency commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities, available for sale amortized cost	6.6	6.6
Available for sale investments in fixed income maturities, Fair Market Value	5.8	5.6
Fixed maturities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities, available for sale amortized cost	4,438.3	4,183.7
Available for sale investments in fixed income maturities, Fair Market Value	$ 4,231.1	$ 3,840.6

Investments - Aggregate Fair Va

Investments - Aggregate Fair Value and Gross Unrealized Loss by Type of Security (Details) $ in Millions	Dec. 31, 2023 USD ($) security	Dec. 31, 2022 USD ($) security Security
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 196.5	$ 2,321.4
Gross unrealized losses, less than twelve months	(2.1)	(145.4)
Over 12 months, Fair Market Value	2,802	1,323.7
Over 12 months, Gross Unrealized Losses	(219.7)	(193.6)
Total, Fair Market Value	2,998.5	3,645.1
Total, Gross Unrealized Losses	$ (221.8)	$ (339)
Number of Securities | security	950	1,166
Fixed income securities — Available for sale
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value		$ 2,269.5
Gross unrealized losses, less than twelve months		(145)
Over 12 months, Fair Market Value		1,323.7
Over 12 months, Gross Unrealized Losses		(193.6)
Total, Fair Market Value		3,593.2
Total, Gross Unrealized Losses		$ (338.6)
Number of Securities | Security		1,165
Short-term investments
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value		$ 51.9
Gross unrealized losses, less than twelve months		(0.4)
Over 12 months, Fair Market Value		0
Over 12 months, Gross Unrealized Losses		0
Total, Fair Market Value		51.9
Total, Gross Unrealized Losses		$ (0.4)
Number of Securities | Security		1
U.S. government
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 105.5	$ 741.1
Gross unrealized losses, less than twelve months	(0.5)	(30.6)
Over 12 months, Fair Market Value	673.3	203.4
Over 12 months, Gross Unrealized Losses	(26.2)	(20.1)
Total, Fair Market Value	778.8	944.5
Total, Gross Unrealized Losses	$ (26.7)	$ (50.7)
Number of Securities | security	74	118
U.S. agency
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 0	$ 7.1
Gross unrealized losses, less than twelve months	0	(0.4)
Over 12 months, Fair Market Value	7.2	1.7
Over 12 months, Gross Unrealized Losses	(0.3)	0
Total, Fair Market Value	7.2	8.8
Total, Gross Unrealized Losses	$ (0.3)	$ (0.4)
Number of Securities | security	1	5
Municipal
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 11.1	$ 133.1
Gross unrealized losses, less than twelve months	(1)	(7.1)
Over 12 months, Fair Market Value	117	16.4
Over 12 months, Gross Unrealized Losses	(4)	(2.3)
Total, Fair Market Value	128.1	149.5
Total, Gross Unrealized Losses	$ (5)	$ (9.4)
Number of Securities | security	54	59
Corporate
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 46.7	$ 1,104.7
Gross unrealized losses, less than twelve months	(0.4)	(77.6)
Over 12 months, Fair Market Value	1,287.2	568.2
Over 12 months, Gross Unrealized Losses	(101.1)	(91.5)
Total, Fair Market Value	1,333.9	1,672.9
Total, Gross Unrealized Losses	$ (101.5)	$ (169.1)
Number of Securities | security	558	701
Non-U.S. government-backed corporate
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 0.2	$ 11.3
Gross unrealized losses, less than twelve months	0	(0.5)
Over 12 months, Fair Market Value	95.5	99
Over 12 months, Gross Unrealized Losses	(5.9)	(8.3)
Total, Fair Market Value	95.7	110.3
Total, Gross Unrealized Losses	$ (5.9)	$ (8.8)
Number of Securities | security	12	16
Non-U.S government
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 32.9	$ 63.8
Gross unrealized losses, less than twelve months	(0.2)	(2.9)
Over 12 months, Fair Market Value	180.1	135.9
Over 12 months, Gross Unrealized Losses	(6.5)	(8.6)
Total, Fair Market Value	213	199.7
Total, Gross Unrealized Losses	$ (6.7)	$ (11.5)
Number of Securities | security	53	45
Agency commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 0	$ 5.6
Gross unrealized losses, less than twelve months	0	(1)
Over 12 months, Fair Market Value	5.8	0
Over 12 months, Gross Unrealized Losses	(0.8)	0
Total, Fair Market Value	5.8	5.6
Total, Gross Unrealized Losses	$ (0.8)	$ (1)
Number of Securities | security	1	1
Residential Mortgage-Backed Securities
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value	$ 0.1
Gross unrealized losses, less than twelve months	0
Over 12 months, Fair Market Value	435.9
Over 12 months, Gross Unrealized Losses	(74.9)
Total, Fair Market Value	436
Total, Gross Unrealized Losses	$ (74.9)
Number of Securities | security	197
Agency mortgage-backed
Debt Securities, Available-for-sale [Line Items]
0-12 months, Fair Market Value		$ 202.8
Gross unrealized losses, less than twelve months		(24.9)
Over 12 months, Fair Market Value		299.1
Over 12 months, Gross Unrealized Losses		(62.8)
Total, Fair Market Value		501.9
Total, Gross Unrealized Losses		$ (87.7)
Number of Securities | security		220

Investments - Additional Inform

Investments - Additional Information (Narrative) (Details)	1 Months Ended	12 Months Ended
	Mar. 31, 2021 USD ($)	Dec. 31, 2023 USD ($) security	Dec. 31, 2022 USD ($) security	Dec. 31, 2021 USD ($)	Aug. 31, 2023 USD ($)	Apr. 01, 2023 USD ($)	May 05, 2022 USD ($)	Apr. 01, 2022 USD ($)	Sep. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 23, 2019 USD ($)	Dec. 20, 2017 USD ($)	Jan. 01, 2017 USD ($)
Gain (Loss) on Securities [Line Items]
Catastrophe bonds, trading, cost		$ 1,600,000	$ 2,900,000
Investment in the period		400,000	2,000,000
Equity method investment, aggregate cost		7,600,000	6,200,000	$ 3,900,000
Other investments, at fair value (2)		209,300,000	221,300,000
Investment in Multiline reinsurer		0	0
Investment funded		0
Available-for-sale investment, allowance for credit loss, increase (decrease)		$ (3,600,000)	$ 200,000
Number of Securities | security		950	1,166
Available-for-sale securities, unrealized loss position		$ 221,800,000	$ 339,000,000
External Credit Rating, Non Investment Grade
Gain (Loss) on Securities [Line Items]
Available-for-sale securities, unrealized loss position		1,200,000
Investment, Identifier [Axis]: Unfunded Commitment, Apollo Real Estate Fund
Gain (Loss) on Securities [Line Items]
Unfunded commitment		4,100,000
Investment, Identifier [Axis]: Unfunded Commitment, Infrastructure fund
Gain (Loss) on Securities [Line Items]
Unfunded commitment		4,000,000
Investment, Identifier [Axis]: Unfunded Commitment, Managed Debt Fund
Gain (Loss) on Securities [Line Items]
Unfunded commitment		6,900,000
Investment, Identifier [Axis]: Unfunded Commitment, Managed lending fund
Gain (Loss) on Securities [Line Items]
Unfunded commitment		1,100,000
Investment, Identifier [Axis]: Unfunded Commitment, Real Estate Fund (Sept 2021)
Gain (Loss) on Securities [Line Items]
Unfunded commitment		2,200,000
Fair Value, Recurring [Member]
Gain (Loss) on Securities [Line Items]
Investments, fair value		117,100,000	134,600,000
Fair Value, Recurring [Member] | Apollo real estate fund
Gain (Loss) on Securities [Line Items]
Other investments, at fair value (2)		23,900,000	25,300,000
Real estate fund | Fair Value, Recurring [Member]
Gain (Loss) on Securities [Line Items]
Investments, fair value		209,300,000	221,300,000
Commercial mortgage loans
Gain (Loss) on Securities [Line Items]
Fair market value, trading		274,900,000	312,100,000
Middle market loans
Gain (Loss) on Securities [Line Items]
Fair market value, trading		84,800,000	106,900,000
Privately-held investments
Gain (Loss) on Securities [Line Items]
Fair market value, trading		475,000,000	533,000,000
Real estate fund (Sept 2021) | Fair Value, Recurring [Member]
Gain (Loss) on Securities [Line Items]
Investments, fair value		39,800,000	40,600,000
Infrastructure fund | Fair Value, Recurring [Member]
Gain (Loss) on Securities [Line Items]
Investments, fair value		10,800,000	8,200,000
Managed Lending Fund (Dec 2020) | Fair Value, Recurring [Member]
Gain (Loss) on Securities [Line Items]
Investments, fair value		15,900,000	12,700,000
Pledge Accounts, Custodian Bank (April 2021) | Fair Value, Recurring [Member]
Gain (Loss) on Securities [Line Items]
Investments, fair value		1,700,000	1,300,000
Managed debt fund | Fair Value, Recurring [Member]
Gain (Loss) on Securities [Line Items]
Investments, fair value		100,000	0
MVI
Gain (Loss) on Securities [Line Items]
Investment in the period	$ 800,000	0	400,000
Equity method investment, aggregate cost		1,000,000	800,000	500,000
Gross realized and unrealized gains (loss)		200,000	0	100,000
Investment in Multiline reinsurer		0	0
Digital Re
Gain (Loss) on Securities [Line Items]
Investment in the period		0	0
Equity method investment, ownership percentage													49%
Equity method investment, aggregate cost		200,000	200,000	200,000									$ 2,300,000
Investment in Multiline reinsurer		0	0
Multi-Line Insurer
Gain (Loss) on Securities [Line Items]
Investment in the period		400,000	1,600,000
Equity method investment, aggregate cost		6,400,000	5,200,000	3,200,000
Other investments, at fair value (2)											$ 5,000,000
Gross realized and unrealized gains (loss)		800,000	(400,000)	$ 200,000
Investment in Multiline reinsurer		$ 0	$ 0
Limited Partner
Gain (Loss) on Securities [Line Items]
Restricted assets								$ 10,000,000	$ 20,000,000			$ 100,000,000
Limited Partner | Real estate fund | Apollo real estate fund
Gain (Loss) on Securities [Line Items]
Restricted assets								$ 30,000,000
Limited Partner | Infrastructure fund
Gain (Loss) on Securities [Line Items]
Restricted assets							$ 15,000,000
Limited Partner | Managed lending fund
Gain (Loss) on Securities [Line Items]
Restricted assets						$ 2,800,000				$ 10,500,000
Limited Partner | Managed debt fund
Gain (Loss) on Securities [Line Items]
Restricted assets					$ 7,000,000

Variable Interest Entities - Na

Variable Interest Entities - Narrative (Details) - Investment	Dec. 31, 2023	Dec. 31, 2022
Payables and Accruals [Abstract]
Number of investments in VIEs	1	1

Fair Value Measurements - Finan

Fair Value Measurements - Financial Assets Measured on Recurring Basis (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments, available for sale, at fair value	$ 14.9	$ 0
Catastrophe Bonds, Fair Value Disclosure	1.6	5.1
Derivatives at fair value	31.7	56.2
Fair market value	4,122.6	3,788.6
Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	2.1	6.3
Commercial mortgage loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	274.9	312.1
Middle market loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	84.8	106.9
Equity securities — Trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)		6.6
Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	18	25.6
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	117.1	134.6
Privately-held Investments trading, at fair value	475
Total	6,410.7	6,100.9
Recurring | Derivatives at fair value | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives at fair value	31.7	56.2
Recurring | Liabilities under derivative contracts | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities under derivative contracts, fair value	(9.3)	(3.2)
Recurring | Liabilities under derivative contracts | Loss Portfolio Transfer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities under derivative contracts, fair value	(16.5)	(31.7)
Recurring | Total fixed income securities — Available for sale | Agency commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	5.8
Recurring | Fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	4,122.6	3,788.6
Recurring | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	93.6	52
Recurring | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,485.7	1,475.5
Catastrophe Bonds, Fair Value Disclosure	1.6	2.9
Recurring | Short-term investments trading at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	2.1
Recurring | Real estate fund
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	209.3	221.3
Recurring | Privately-held investments, Available for Sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments, available for sale, at fair value	14.9
Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments trading, at fair value	0
Total	1,748.4	1,428.7
Recurring | Level 1 | Derivatives at fair value | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives at fair value		0
Recurring | Level 1 | Liabilities under derivative contracts | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities under derivative contracts, fair value	0	0
Recurring | Level 1 | Liabilities under derivative contracts | Loss Portfolio Transfer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities under derivative contracts, fair value	0	0
Recurring | Level 1 | Total fixed income securities — Available for sale | Agency commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0
Recurring | Level 1 | Fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,403	1,098.3
Recurring | Level 1 | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	86.7	51.9
Recurring | Level 1 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	258.5	272.2
Catastrophe Bonds, Fair Value Disclosure	0	0
Recurring | Level 1 | Short-term investments trading at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0.2
Recurring | Level 1 | Real estate fund
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 1 | Privately-held investments, Available for Sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments, available for sale, at fair value	0
Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments trading, at fair value	0
Total	3,979.6	3,949.6
Recurring | Level 2 | Derivatives at fair value | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives at fair value	31.7	56.2
Recurring | Level 2 | Liabilities under derivative contracts | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities under derivative contracts, fair value	(9.3)	(3.2)
Recurring | Level 2 | Liabilities under derivative contracts | Loss Portfolio Transfer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities under derivative contracts, fair value		0
Recurring | Level 2 | Total fixed income securities — Available for sale | Agency commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	5.8
Recurring | Level 2 | Fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	2,719.6	2,690.3
Recurring | Level 2 | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	6.9	0.1
Recurring | Level 2 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,227.2	1,203.3
Catastrophe Bonds, Fair Value Disclosure	1.6	2.9
Recurring | Level 2 | Short-term investments trading at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1.9
Recurring | Level 2 | Real estate fund
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 2 | Privately-held investments, Available for Sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments, available for sale, at fair value	0
Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments trading, at fair value	475
Total	473.4	501.3
Change in unrealized investment gains (losses)	(15.7)	(4.6)
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	489.9	533
Recurring | Level 3 | Liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value	(16.5)	(31.7)	$ 0
Liability purchases	0	(17.2)
Liability transfers	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3	0	0
Liability sales	0	0
Liability, increase (decrease) in fv included in net income	15.2	(14.5)
Change in unrealized gains or losses	15.2	0
Recurring | Level 3 | Loss Portfolio Transfer | Liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value	(16.5)	(31.7)	0
Liability purchases	0	(17.2)
Liability transfers	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3	0	0
Liability sales	0	0
Liability, increase (decrease) in fv included in net income	15.2	(14.5)
Change in unrealized gains or losses	15.2	0
Recurring | Level 3 | Derivatives at fair value | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives at fair value		0
Recurring | Level 3 | Liabilities under derivative contracts | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities under derivative contracts, fair value	0	0
Recurring | Level 3 | Liabilities under derivative contracts | Loss Portfolio Transfer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities under derivative contracts, fair value	(16.5)	(31.7)
Recurring | Level 3 | Assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Purchases and issuances	113.7	378.2
Transfers in/(out)	5.3	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	(12.1)	0
Settlements and sales	(137.2)	(147.4)
Increase (decrease) in fair value included in net income	(12.8)	(4.9)
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)		533	307.1
Recurring | Level 3 | Total fixed income securities — Available for sale | Agency commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0
Recurring | Level 3 | Fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 3 | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 3 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Catastrophe Bonds, Fair Value Disclosure	0	0
Recurring | Level 3 | Short-term investments trading at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0
Recurring | Level 3 | Real estate fund
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 3 | Commercial mortgage loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Change in unrealized investment gains (losses)	(17.9)	(0.5)
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	274.9	312.1
Recurring | Level 3 | Commercial mortgage loans | Assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Purchases and issuances	40.6	215.7
Transfers in/(out)	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0	0
Settlements and sales	(61.5)	(113.1)
Increase (decrease) in fair value included in net income	(16.3)	(2)
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)		312.1	211.5
Recurring | Level 3 | Middle market loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Change in unrealized investment gains (losses)	0.5	(2.1)
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	84.8	106.9
Recurring | Level 3 | Middle market loans [Member] | Assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Purchases and issuances	18.3	61.8
Transfers in/(out)	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0	0
Settlements and sales	(41.9)	(19.3)
Increase (decrease) in fair value included in net income	1.5	(0.9)
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)		106.9	65.3
Recurring | Level 3 | Global corporate securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Change in unrealized investment gains (losses)	(0.2)	0
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	14.4	15
Recurring | Level 3 | Global corporate securities | Assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Purchases and issuances	0	15.1
Transfers in/(out)	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0	0
Settlements and sales	(0.4)	0
Increase (decrease) in fair value included in net income	(0.2)	(0.1)
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)		15	0
Recurring | Level 3 | Equity securities — Trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Change in unrealized investment gains (losses)	0	0
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	0	6.6
Recurring | Level 3 | Equity securities — Trading | Assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Purchases and issuances	0	5.5
Transfers in/(out)	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	(6.6)	0
Settlements and sales	0	(2.4)
Increase (decrease) in fair value included in net income	0	(0.1)
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)		6.6	3.6
Recurring | Level 3 | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Change in unrealized investment gains (losses)	0	0
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	18	25.6
Recurring | Level 3 | Short-term investments | Assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Purchases and issuances	18.2	25.6
Transfers in/(out)	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0	0
Settlements and sales	(25.8)	0
Increase (decrease) in fair value included in net income	0	0
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)		25.6	0
Recurring | Level 3 | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Change in unrealized investment gains (losses)	1.7	(2)
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	82.9	66.8
Recurring | Level 3 | Asset-backed | Assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Purchases and issuances	21.9	54.5
Transfers in/(out)	5.3	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	(5.5)	0
Settlements and sales	(7.6)	(12.6)
Increase (decrease) in fair value included in net income	2	(1.8)
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)		66.8	$ 26.7
Recurring | Level 3 | Privately-held investments, Available for Sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments, available for sale, at fair value	14.9
U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	1,202.6	953
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	245.5	261.6
U.S. government | Recurring | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,202.6	953
U.S. government | Recurring | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	245.5	261.6
U.S. government | Recurring | Level 1 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,202.6	953
U.S. government | Recurring | Level 1 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	245.5	261.6
U.S. government | Recurring | Level 2 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
U.S. government | Recurring | Level 2 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
U.S. government | Recurring | Level 3 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
U.S. government | Recurring | Level 3 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
U.S. agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	7.2	8.8
U.S. agency | Recurring | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	7.2	8.8
U.S. agency | Recurring | Level 1 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
U.S. agency | Recurring | Level 2 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	7.2	8.8
U.S. agency | Recurring | Level 3 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	128.1	149.5
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	3.1	3.6
Municipal | Recurring | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	128.1	149.5
Municipal | Recurring | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	3.1	3.6
Municipal | Recurring | Level 1 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Municipal | Recurring | Level 1 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Municipal | Recurring | Level 2 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	128.1	149.5
Municipal | Recurring | Level 2 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	3.1	3.6
Municipal | Recurring | Level 3 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Municipal | Recurring | Level 3 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	1,959.3	1,845
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	171.5	162.1
Corporate | Recurring | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,959.3	1,845
Corporate | Recurring | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	171.5	162.1
Corporate | Recurring | Level 1 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Corporate | Recurring | Level 1 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Corporate | Recurring | Level 2 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,959.3	1,845
Corporate | Recurring | Level 2 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	171.5	162.1
Corporate | Recurring | Level 3 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Corporate | Recurring | Level 3 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Non-U.S. government-backed corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	100.7	110.4
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	8.3	11.6
Non-U.S. government-backed corporate | Recurring | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	100.7	110.4
Non-U.S. government-backed corporate | Recurring | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	8.3	11.6
Non-U.S. government-backed corporate | Recurring | Level 1 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Non-U.S. government-backed corporate | Recurring | Level 1 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Non-U.S. government-backed corporate | Recurring | Level 2 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	100.7	110.4
Non-U.S. government-backed corporate | Recurring | Level 2 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	8.3	11.6
Non-U.S. government-backed corporate | Recurring | Level 3 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Non-U.S. government-backed corporate | Recurring | Level 3 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Non-U.S government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	273.8	213.6
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	34.8	30.4
Non-U.S government | Recurring | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	273.8	213.6
Non-U.S government | Recurring | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	34.8	30.4
Non-U.S government | Recurring | Level 1 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	200.4	145.3
Non-U.S government | Recurring | Level 1 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	13	10.6
Non-U.S government | Recurring | Level 2 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	73.4	68.3
Non-U.S government | Recurring | Level 2 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	21.8	19.8
Non-U.S government | Recurring | Level 3 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Non-U.S government | Recurring | Level 3 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	908.2	896.5
Asset-backed | Recurring | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	908.2	896.5
Asset-backed | Recurring | Level 1 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Asset-backed | Recurring | Level 2 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	908.2	896.5
Asset-backed | Recurring | Level 3 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Agency commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	5.8	5.6
Agency commercial mortgage-backed securities | Recurring | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		5.6
Agency commercial mortgage-backed securities | Recurring | Level 1 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		0
Agency commercial mortgage-backed securities | Recurring | Level 2 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		5.6
Agency commercial mortgage-backed securities | Recurring | Level 3 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		0
Residential Mortgage-Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	445.1
Residential Mortgage-Backed Securities | Recurring | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	445.1
Residential Mortgage-Backed Securities | Recurring | Level 1 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0
Residential Mortgage-Backed Securities | Recurring | Level 2 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	445.1
Residential Mortgage-Backed Securities | Recurring | Level 3 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0
Agency mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	445.1	502.7
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)	22.2	21.4
Agency mortgage-backed | Recurring | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		502.7
Agency mortgage-backed | Recurring | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	22.2	21.4
Agency mortgage-backed | Recurring | Level 1 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		0
Agency mortgage-backed | Recurring | Level 1 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Agency mortgage-backed | Recurring | Level 2 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		502.7
Agency mortgage-backed | Recurring | Level 2 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	22.2	21.4
Agency mortgage-backed | Recurring | Level 3 | Total fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		0
Agency mortgage-backed | Recurring | Level 3 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
High yield loans | Recurring | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	92.1	88.3
High yield loans | Recurring | Level 1 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
High yield loans | Recurring | Level 2 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	92.1	88.3
High yield loans | Recurring | Level 3 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Held for trading financial assets, at fair value | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments trading, at fair value		533
Held for trading financial assets, at fair value | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments trading, at fair value		0
Held for trading financial assets, at fair value | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments trading, at fair value		0
Held for trading financial assets, at fair value | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments trading, at fair value		533
Short-term investments trading at fair value | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		6.3
Short-term investments trading at fair value | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		6.3
Short-term investments trading at fair value | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		0
Short-term investments trading at fair value | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		0
Asset backed securities, privately-held afs | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	14.9
Change in unrealized investment gains (losses)	0.2
Asset backed securities, privately-held afs | Recurring | Level 3 | Assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value		$ 0
Purchases and issuances	14.7
Transfers in/(out)	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0
Settlements and sales	0
Increase (decrease) in fair value included in net income	$ 0.2

Fair Value Measurements Fair Va

Fair Value Measurements Fair Value Measurements - Reconciliation of Level 3 Assets and Liabilities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]
Beginning of the year	$ 3,788.6
End of the year	$ 4,122.6	$ 3,788.6
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Accumulated other comprehensive (loss)
Commercial mortgage loans
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]
Balance at the beginning of the year	$ 312.1
Balance at the end of the year	274.9	312.1
Middle market loans
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]
Balance at the beginning of the year	106.9
Balance at the end of the year	84.8	106.9
Equity securities
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]
Balance at the beginning of the year	6.6
Balance at the end of the year		6.6
Short-term investments
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]
Balance at the beginning of the year	25.6
Balance at the end of the year	18	25.6
Recurring | Level 3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Change in unrealized investment gains (losses)	(15.7)	(4.6)
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]
Balance at the beginning of the year	533
Balance at the end of the year	489.9	533
Recurring | Level 3 | Liability
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]
Balance at the beginning of the year	(31.7)	0
Liability purchases	0	(17.2)
Liability transfers	0	0
Liability sales	0	0
Liability, increase (decrease) in fv included in net income	15.2	(14.5)
Balance at the end of the period	(16.5)	(31.7)
Change in unrealized gains or losses	15.2	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3	0	0
Recurring | Level 3 | Liability | Loss Portfolio Transfer
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]
Balance at the beginning of the year	(31.7)	0
Liability purchases	0	(17.2)
Liability transfers	0	0
Liability sales	0	0
Liability, increase (decrease) in fv included in net income	15.2	(14.5)
Balance at the end of the period	(16.5)	(31.7)
Change in unrealized gains or losses	15.2	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3	0	0
Recurring | Commercial mortgage loans | Level 3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Change in unrealized investment gains (losses)	(17.9)	(0.5)
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]
Balance at the beginning of the year	312.1
Balance at the end of the year	274.9	312.1
Recurring | Middle market loans | Level 3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Change in unrealized investment gains (losses)	0.5	(2.1)
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]
Balance at the beginning of the year	106.9
Balance at the end of the year	84.8	106.9
Recurring | Asset-backed | Level 3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Change in unrealized investment gains (losses)	1.7	(2)
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]
Balance at the beginning of the year	66.8
Balance at the end of the year	82.9	66.8
Recurring | Global corporate securities | Level 3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Change in unrealized investment gains (losses)	(0.2)	0
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]
Balance at the beginning of the year	15
Balance at the end of the year	14.4	15
Recurring | Equity securities | Level 3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Change in unrealized investment gains (losses)	0	0
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]
Balance at the beginning of the year	6.6
Balance at the end of the year	0	6.6
Recurring | Short-term investments | Level 3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Change in unrealized investment gains (losses)	0	0
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]
Balance at the beginning of the year	25.6
Balance at the end of the year	18	25.6
Assets | Recurring | Level 3
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]
Balance at the beginning of the year	533	307.1
Purchases and issuances	113.7	378.2
Transfers in/(out)	5.3	0
Settlements and sales	(137.2)	(147.4)
Increase (decrease) in fair value included in net income	(12.8)	(4.9)
Balance at the end of the year		533
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	(12.1)	0
Assets | Recurring | Commercial mortgage loans | Level 3
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]
Balance at the beginning of the year	312.1	211.5
Purchases and issuances	40.6	215.7
Transfers in/(out)	0	0
Settlements and sales	(61.5)	(113.1)
Increase (decrease) in fair value included in net income	(16.3)	(2)
Balance at the end of the year		312.1
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0	0
Assets | Recurring | Middle market loans | Level 3
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]
Balance at the beginning of the year	106.9	65.3
Purchases and issuances	18.3	61.8
Transfers in/(out)	0	0
Settlements and sales	(41.9)	(19.3)
Increase (decrease) in fair value included in net income	1.5	(0.9)
Balance at the end of the year		106.9
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0	0
Assets | Recurring | Asset-backed | Level 3
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]
Balance at the beginning of the year	66.8	26.7
Purchases and issuances	21.9	54.5
Transfers in/(out)	5.3	0
Settlements and sales	(7.6)	(12.6)
Increase (decrease) in fair value included in net income	2	(1.8)
Balance at the end of the year		66.8
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	(5.5)	0
Assets | Recurring | Global corporate securities | Level 3
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]
Balance at the beginning of the year	15	0
Purchases and issuances	0	15.1
Transfers in/(out)	0	0
Settlements and sales	(0.4)	0
Increase (decrease) in fair value included in net income	(0.2)	(0.1)
Balance at the end of the year		15
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0	0
Assets | Recurring | Equity securities | Level 3
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]
Balance at the beginning of the year	6.6	3.6
Purchases and issuances	0	5.5
Transfers in/(out)	0	0
Settlements and sales	0	(2.4)
Increase (decrease) in fair value included in net income	0	(0.1)
Balance at the end of the year		6.6
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	(6.6)	0
Assets | Recurring | Short-term investments | Level 3
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]
Balance at the beginning of the year	25.6	0
Purchases and issuances	18.2	25.6
Transfers in/(out)	0	0
Settlements and sales	(25.8)	0
Increase (decrease) in fair value included in net income	0	0
Balance at the end of the year		25.6
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	$ 0	$ 0

Fair Value Measurements - Input

Fair Value Measurements - Inputs Used (Details) $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross written premiums	$ 3,967.6	$ 4,338.7	$ 3,938.4
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments trading, at fair value	475
Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments trading, at fair value	475
Total, Privately-held Investments	$ 489.9
Discounted cash flow | Commercial mortgage loans | Minimum | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs	0.038
Discounted cash flow | Commercial mortgage loans | Maximum | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs	0.200
Discounted cash flow | Commercial mortgage loans | Weighted Average | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs	0.077
Discounted cash flow | Commercial mortgage loans | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments trading, at fair value	$ 274.9
Discounted cash flow | Middle market loans | Minimum | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs	0.077
Discounted cash flow | Middle market loans | Maximum | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs	0.185
Discounted cash flow | Middle market loans | Weighted Average | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs	0.110
Discounted cash flow | Middle market loans | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments trading, at fair value	$ 84.8
Discounted cash flow | Asset-backed | Minimum | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs	0.059
Discounted cash flow | Asset-backed | Maximum | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs	0.097
Discounted cash flow | Asset-backed | Weighted Average | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs	0.070
Discounted cash flow | Asset-backed | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments trading, at fair value	$ 82.9
Discounted cash flow | Global corporate securities | Minimum | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs	0.066
Discounted cash flow | Global corporate securities | Maximum | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs	0.066
Discounted cash flow | Global corporate securities | Weighted Average | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs	0.066
Discounted cash flow | Global corporate securities | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments trading, at fair value	$ 14.4
Discounted cash flow | Short-term investments | Minimum | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs	0.093
Discounted cash flow | Short-term investments | Maximum | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs	0.185
Discounted cash flow | Short-term investments | Weighted Average | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs	0.093
Discounted cash flow | Short-term investments | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments trading, at fair value	$ 18
Valuation, Market Approach | Asset-backed | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments trading, at fair value	$ 14.9

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($) quotesPerInvestment	Dec. 31, 2022 USD ($) quotesPerInvestment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Quotes per fixed income investment | quotesPerInvestment	2.9	2.9
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments trading, at fair value	$ 475
Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Privately-held Investments trading, at fair value	475
Total, Privately-held Investments	489.9
Assets | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	(12.1)	$ 0
Transfers in/(out)	5.3	0
Settlements and sales	(137.2)	(147.4)
Equity securities — Trading | Assets | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	(6.6)	0
Transfers in/(out)	0	0
Settlements and sales	$ 0	$ (2.4)

Reinsurance - Summary of Assume

Reinsurance - Summary of Assumed and Ceded Reinsurance on Premiums Written, Premiums Earned and Insurance Losses and Loss Adjustment Expenses (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Premiums written:
Direct	$ 2,446.6	$ 2,531.7	$ 2,341.4
Assumed	1,521	1,807	1,597
Ceded	(1,385.7)	(1,442.7)	(1,350.7)
Net premiums written	2,581.9	2,896	2,587.7
Premiums earned:
Direct	2,444.8	2,370.8	2,139.1
Assumed	1,562	1,617.2	1,479.2
Ceded	(1,392.3)	(1,299.3)	(1,207.8)
Net premiums earned	2,614.5	2,688.7	2,410.5
Insurance losses and loss adjustment expenses:
Direct	1,644.5	1,574.2	1,499.8
Assumed	707.2	939.5	1,000.6
Ceded	(798.7)	(833.7)	(807.1)
Net insurance losses and loss adjustment expenses	$ 1,553	$ 1,680	$ 1,693.3

Reinsurance - Additional Inform

Reinsurance - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Reinsurance Recoverable, Allowance for Credit Loss	$ 3.7	$ 3.7	$ 3.3
Reinsurance Recoverable, Allowance for Credit Loss, Period Increase (Decrease)	$ 0	$ 0.4	$ 0.5

Derivative Contracts - Fair Val

Derivative Contracts - Fair Value of Derivative Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Not Designated as Hedging Instrument | Foreign exchange contracts
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	$ 1,802.9	$ 1,675.3
Cash collateral	3.4	3.7
Not Designated as Hedging Instrument | Derivative Financial Instruments, Assets [Member] | Foreign exchange contracts
Derivatives, Fair Value [Line Items]
Fair Value	31.4	41.9
Derivative, Notional Amount	1,262.1	945.8
Not Designated as Hedging Instrument | Derivative liabilities | Foreign exchange contracts
Derivatives, Fair Value [Line Items]
Fair Value	(9.3)	(3.2)
Derivative, Notional Amount	540.8	729.5
Not Designated as Hedging Instrument | Derivative liabilities | Embedded Derivative Financial Instruments
Derivatives, Fair Value [Line Items]
Fair Value	(16.5)	(31.7)
Derivative, Notional Amount	0	0
Designated as Hedging Instrument | Derivative Financial Instruments, Assets [Member] | Foreign exchange contracts
Derivatives, Fair Value [Line Items]
Fair Value	0.3	14.3
Derivative, Notional Amount	$ 76.9	$ 109.7

Derivative Contracts - Gain_(Lo

Derivative Contracts - Gain/(Loss) Recognized in Income on Derivative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Not Designated as Hedging Instrument | Foreign exchange contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) to net income from derivative instruments	$ 10.9	$ (66)
Not Designated as Hedging Instrument | Loss portfolio transfer liability, embedded derivative
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) to net income from derivative instruments	15.2	(14.5)
Designated as Hedging Instrument | Foreign exchange contracts | General, administrative and corporate expenses
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on foreign currency fair value hedge derivatives	(8.1)	5.9
Designated as Hedging Instrument | Foreign exchange contracts | Net change from current period hedged transactions
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on foreign currency fair value hedge derivatives	$ (14)	$ 15.4

Derivative Contracts - Addition

Derivative Contracts - Additional Information (Details) - USD ($) $ in Millions		12 Months Ended
	Oct. 01, 2022	Dec. 31, 2023	Dec. 31, 2022
Reinsurance Contracts [Axis]: Funds withheld credited with interest for periods after October 1, 2022
Derivative Instruments, Gain (Loss) [Line Items]
Reinsurance Retention Policy, Retrocession Premium, Interest Rate	1.75%
Level 3 | Liability | Fair Value, Recurring [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Liability, increase (decrease) in fv included in net income		$ 15.2	$ (14.5)
Foreign exchange contracts | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Purchase of US and foreign exchange		1,802.9	1,675.3
Gain (loss) to net income from derivative instruments		10.9	(66)
Foreign exchange contracts | Not Designated as Hedging Instrument | Derivative liabilities
Derivative Instruments, Gain (Loss) [Line Items]
Purchase of US and foreign exchange		540.8	729.5
Foreign exchange contracts | Net change from current period hedged transactions | Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on foreign currency fair value hedge derivatives		(14)	15.4
Foreign exchange contracts | General, administrative and corporate expenses | Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on foreign currency fair value hedge derivatives		(8.1)	5.9
Foreign exchange contracts | General, administrative and corporate expenses | Designated as Hedging Instrument | Forecast
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on foreign currency fair value hedge derivatives		0.3
Loss Portfolio Transfer | Level 3 | Liability | Fair Value, Recurring [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Liability, increase (decrease) in fv included in net income		$ 15.2	$ (14.5)

Deferred Policy Acquisition C_3

Deferred Policy Acquisition Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance at the beginning of the period	$ 319	$ 290.8
Acquisition costs deferred	357.4	460
Amortization of deferred acquisition costs	(380.2)	(431.8)	$ (414.1)
Balance at the end of the period	$ 296.2	$ 319	$ 290.8

Reserves for Losses and Adjus_3

Reserves for Losses and Adjustment Expenses - Reconciliation of Beginning and Ending Consolidated Loss and Loss Adjustment Expenses (LAE) Reserves (Details) - USD ($) $ in Millions		12 Months Ended
	Jan. 10, 2022	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	May 30, 2022	Dec. 31, 2020	Mar. 02, 2020
Insurance [Abstract]
Provision for losses and LAE, beginning balance		$ 7,710.9	$ 7,611.8	$ 7,165.3
Less reinsurance recoverable			(4,897.7)	(3,298.1)		$ (3,195.2)
Net loss and LAE expenses disposed (1)		0	1,840.1	0
Net losses and LAE reserves, beginning balance		2,813.2	4,313.7	3,970.1
Movement in net provision for losses and LAE for claims incurred:
Current year		1,492.2	1,651.9	1,648.2
Prior years		60.8	28.1
Total incurred		1,553	1,680	1,693.3
Net Losses and LAE payments for claims incurred:
Current year		(161.1)	(192.7)	(729.1)
Prior years		(1,011.9)	(1,098.4)	(580.7)
Total paid		(1,173)	(1,291.1)	(1,309.8)
Foreign exchange (gains)/losses		39.6	(49.3)	(39.9)
Net losses and LAE reserves, ending balance		3,232.8	2,813.2	4,313.7
Plus reinsurance recoverable on unpaid losses at the end of the year		4,577.8	4,897.7	3,298.1
Provision for losses and LAE at the end of the year		7,810.6	7,710.9	7,611.8
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Reinsurance recoverable on unpaid losses		4,577.8	4,897.7
Plus reinsurance recoverable on unpaid losses at the end of the year		4,577.8	4,897.7	3,298.1
Reinsurance Contracts [Axis]: LPT
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Reinsurance recoverable total					$ (2,610.1)
Reinsurance recoverables, premium paid and recognized					$ 1,840.1
Reinsurance Contracts [Axis]: Retroactive Reinsurance
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Reinsurance recoverable on unpaid losses		1,627.4	2,132
Insurance
Insurance [Abstract]
Provision for losses and LAE, beginning balance		1,452.5	2,165.3
Net losses and LAE reserves, beginning balance		1,452.5	2,165.3
Net Losses and LAE payments for claims incurred:
Net losses and LAE reserves, ending balance		1,859.7	1,452.5	2,165.3
Provision for losses and LAE at the end of the year		$ 1,859.7	$ 1,452.5	$ 2,165.3
Enstar
Net Losses and LAE payments for claims incurred:
Plus reinsurance recoverable on unpaid losses at the end of the year							$ 770
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Plus reinsurance recoverable on unpaid losses at the end of the year							$ 770
Enstar | Reinsurance Contracts [Axis]: LPT net losses reinsured
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Reinsurance, Excess Retention, Amount Reinsured, Per Event	$ 3,120

Reserves for Losses and Adjus_4

Reserves for Losses and Adjustment Expenses - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Insurance [Abstract]
Provision for losses and LAE for claims incurred	$ 60.8	$ 28.1
Financial Guarantee Insurance Contracts, Premium Received over Contract Period [Line Items]
Prior years	$ 60.8	$ 28.1

Reserves for Losses and Adjus_5

Reserves for Losses and Adjustment Expenses - Reconciliation of Reinsurance Recoverables on Unpaid Losses (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	May 30, 2022
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Reinsurance recoverable for unpaid claims under retroactive reinsurance, beginning balance	$ 4,897.7
Reinsurance recoverable for unpaid claims under retroactive reinsurance, beginning balance, Ending Balance	4,577.8	$ 4,897.7
Provision for losses and LAE for claims incurred	60.8	28.1
Reinsurance Contracts [Axis]: LPT
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Reinsurance recoverables, premium paid and recognized			$ 1,840.1
Reinsurance Contracts [Axis]: Retroactive Reinsurance
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Reinsurance recoverable for unpaid claims under retroactive reinsurance, beginning balance	2,132
Reinsurance recoverable for unpaid claims under retroactive reinsurance, beginning balance, Ending Balance	$ 1,627.4	$ 2,132

Reserves for Losses and Adjus_6

Reserves for Losses and Adjustment Expenses Short-duration Insurance Contracts, Claims by Accident Year (Details)	Dec. 31, 2023 USD ($) Integer	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Claims Development [Line Items]
Liability for claims and claim adjustment expenses, net of reinsurance	$ 3,629,600,000	$ 3,151,600,000
Property Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	1,845,700,000
Cumulative paid claims and allocated claim adjustment expense, Net	1,574,900,000
All outstanding liabilities for 2013 and subsequent years, net of reinsurance	270,800,000
All outstanding liabilities before 2013, net of reinsurance	0
Liability for claims and claim adjustment expenses, net of reinsurance	270,800,000	259,000,000
Casualty Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	1,349,900,000
Cumulative paid claims and allocated claim adjustment expense, Net	713,400,000
All outstanding liabilities for 2013 and subsequent years, net of reinsurance	636,500,000
All outstanding liabilities before 2013, net of reinsurance	0
Liability for claims and claim adjustment expenses, net of reinsurance	636,500,000	457,300,000
Marine, Aviation and Energy Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	1,577,100,000
Cumulative paid claims and allocated claim adjustment expense, Net	1,382,600,000
All outstanding liabilities for 2013 and subsequent years, net of reinsurance	194,500,000
All outstanding liabilities before 2013, net of reinsurance	0
Liability for claims and claim adjustment expenses, net of reinsurance	194,500,000	163,700,000
Financial and Professional Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	2,051,400,000
Cumulative paid claims and allocated claim adjustment expense, Net	1,213,800,000
All outstanding liabilities for 2013 and subsequent years, net of reinsurance	837,600,000
All outstanding liabilities before 2013, net of reinsurance	0
Liability for claims and claim adjustment expenses, net of reinsurance	837,600,000	671,900,000
Property Catastrophe and Other Property Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	2,882,400,000
Cumulative paid claims and allocated claim adjustment expense, Net	2,320,100,000
All outstanding liabilities for 2013 and subsequent years, net of reinsurance	562,300,000
All outstanding liabilities before 2013, net of reinsurance	0
Liability for claims and claim adjustment expenses, net of reinsurance	562,300,000	677,900,000
Casualty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	1,550,100,000
Cumulative paid claims and allocated claim adjustment expense, Net	816,500,000
All outstanding liabilities for 2013 and subsequent years, net of reinsurance	733,600,000
All outstanding liabilities before 2013, net of reinsurance	0
Liability for claims and claim adjustment expenses, net of reinsurance	733,600,000	571,600,000
Specialty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	2,314,000,000
Cumulative paid claims and allocated claim adjustment expense, Net	1,919,700,000
All outstanding liabilities for 2013 and subsequent years, net of reinsurance	394,300,000
All outstanding liabilities before 2013, net of reinsurance	0
Liability for claims and claim adjustment expenses, net of reinsurance	394,300,000	350,200,000
Accident Year 2014 | Property Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	132,800,000	132,800,000	$ 125,800,000	$ 131,200,000	$ 131,800,000	$ 133,300,000	$ 134,100,000	$ 133,500,000	$ 156,300,000	$ 164,300,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	10,037
Cumulative paid claims and allocated claim adjustment expense, Net	$ 132,800,000	132,800,000	130,900,000	129,800,000	128,500,000	127,100,000	123,100,000	113,500,000	86,000,000	40,200,000
Accident Year 2014 | Casualty Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	107,000,000	107,000,000	141,400,000	135,100,000	138,100,000	134,200,000	127,100,000	137,100,000	125,300,000	142,900,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	3,962
Cumulative paid claims and allocated claim adjustment expense, Net	$ 107,000,000	107,000,000	106,800,000	108,200,000	95,600,000	71,900,000	58,800,000	32,200,000	13,100,000	2,600,000
Accident Year 2014 | Marine, Aviation and Energy Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	271,300,000	271,300,000	312,400,000	301,000,000	311,700,000	304,800,000	309,200,000	297,400,000	312,300,000	308,800,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	4,056
Cumulative paid claims and allocated claim adjustment expense, Net	$ 271,300,000	271,300,000	269,000,000	262,000,000	249,800,000	231,600,000	208,900,000	188,300,000	116,300,000	53,300,000
Accident Year 2014 | Financial and Professional Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	98,800,000	98,800,000	121,700,000	120,300,000	119,200,000	130,200,000	119,000,000	128,600,000	130,100,000	134,300,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	794
Cumulative paid claims and allocated claim adjustment expense, Net	$ 98,800,000	98,800,000	99,300,000	91,700,000	84,900,000	79,200,000	71,800,000	53,200,000	30,500,000	2,900,000
Accident Year 2014 | Property Catastrophe and Other Property Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	137,100,000	137,100,000	139,700,000	141,100,000	141,100,000	145,800,000	145,700,000	157,200,000	172,800,000	185,800,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	901
Cumulative paid claims and allocated claim adjustment expense, Net	$ 137,100,000	137,100,000	137,400,000	137,500,000	135,900,000	137,300,000	133,800,000	124,500,000	98,700,000	36,800,000
Accident Year 2014 | Casualty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	131,000,000	131,000,000	189,800,000	199,900,000	204,000,000	201,000,000	207,500,000	214,200,000	205,800,000	203,400,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	1,866
Cumulative paid claims and allocated claim adjustment expense, Net	$ 131,000,000	131,000,000	131,000,000	124,400,000	106,800,000	86,000,000	59,900,000	37,600,000	13,700,000	2,500,000
Accident Year 2014 | Specialty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	104,000,000	104,000,000	116,400,000	119,300,000	123,500,000	124,900,000	122,100,000	130,900,000	139,000,000	150,700,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	618
Cumulative paid claims and allocated claim adjustment expense, Net	$ 104,000,000	104,000,000	104,100,000	103,600,000	101,900,000	99,000,000	88,700,000	80,600,000	55,900,000	$ 16,400,000
Accident Year 2015 | Property Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	193,400,000	193,400,000	181,900,000	197,500,000	200,500,000	200,000,000	197,700,000	203,000,000	237,300,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	11,624
Cumulative paid claims and allocated claim adjustment expense, Net	$ 193,400,000	193,400,000	193,400,000	192,400,000	193,800,000	177,500,000	168,400,000	141,000,000	56,900,000
Accident Year 2015 | Casualty Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	180,500,000	180,500,000	257,900,000	232,300,000	234,000,000	201,300,000	183,900,000	221,300,000	201,300,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	4,811
Cumulative paid claims and allocated claim adjustment expense, Net	$ 180,500,000	180,500,000	180,500,000	166,200,000	137,200,000	91,800,000	56,000,000	16,800,000	3,100,000
Accident Year 2015 | Marine, Aviation and Energy Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	267,300,000	267,300,000	318,300,000	311,100,000	308,300,000	284,800,000	280,100,000	297,600,000	295,400,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	4,067
Cumulative paid claims and allocated claim adjustment expense, Net	$ 267,300,000	267,300,000	268,400,000	256,500,000	220,900,000	193,000,000	173,300,000	122,700,000	44,700,000
Accident Year 2015 | Financial and Professional Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	145,900,000	145,900,000	196,700,000	184,600,000	189,800,000	188,700,000	184,600,000	174,800,000	173,300,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	1,083
Cumulative paid claims and allocated claim adjustment expense, Net	$ 145,900,000	145,900,000	150,900,000	138,400,000	109,500,000	89,100,000	69,900,000	43,300,000	13,700,000
Accident Year 2015 | Property Catastrophe and Other Property Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	155,300,000	155,300,000	176,800,000	170,400,000	170,300,000	154,800,000	175,300,000	185,100,000	211,300,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	1,051
Cumulative paid claims and allocated claim adjustment expense, Net	$ 155,300,000	155,300,000	158,600,000	157,100,000	154,800,000	137,800,000	125,400,000	94,200,000	35,800,000
Accident Year 2015 | Casualty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	116,500,000	116,500,000	195,200,000	205,200,000	209,000,000	211,400,000	208,900,000	199,200,000	192,500,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	2,067
Cumulative paid claims and allocated claim adjustment expense, Net	$ 116,500,000	116,500,000	116,500,000	108,000,000	89,000,000	65,200,000	38,100,000	17,800,000	3,400,000
Accident Year 2015 | Specialty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	132,100,000	132,100,000	151,100,000	151,400,000	155,900,000	157,300,000	162,700,000	168,000,000	164,700,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	774
Cumulative paid claims and allocated claim adjustment expense, Net	$ 132,100,000	132,100,000	132,600,000	132,900,000	129,900,000	120,900,000	103,300,000	55,800,000	$ 17,500,000
Accident Year 2016 | Property Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	233,400,000	233,400,000	234,200,000	245,300,000	244,000,000	242,600,000	247,700,000	236,800,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	10,844
Cumulative paid claims and allocated claim adjustment expense, Net	$ 233,400,000	233,400,000	234,900,000	231,100,000	221,900,000	200,200,000	167,800,000	66,500,000
Accident Year 2016 | Casualty Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	124,300,000	124,300,000	243,500,000	198,900,000	187,800,000	181,300,000	186,000,000	215,000,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	4,815
Cumulative paid claims and allocated claim adjustment expense, Net	$ 124,300,000	124,300,000	123,300,000	108,400,000	81,500,000	39,600,000	22,500,000	4,100,000
Accident Year 2016 | Marine, Aviation and Energy Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	197,900,000	197,900,000	220,300,000	218,500,000	228,700,000	228,300,000	229,500,000	259,500,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	4,422
Cumulative paid claims and allocated claim adjustment expense, Net	$ 197,900,000	197,900,000	193,300,000	190,400,000	163,800,000	142,100,000	82,300,000	30,900,000
Accident Year 2016 | Financial and Professional Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	134,300,000	134,300,000	186,100,000	184,600,000	201,200,000	215,400,000	210,900,000	190,100,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	1,241
Cumulative paid claims and allocated claim adjustment expense, Net	$ 134,300,000	134,300,000	130,100,000	125,500,000	129,600,000	101,500,000	71,000,000	15,000,000
Accident Year 2016 | Property Catastrophe and Other Property Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	228,600,000	228,600,000	230,700,000	239,200,000	243,800,000	265,000,000	266,300,000	266,400,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	1,303
Cumulative paid claims and allocated claim adjustment expense, Net	$ 228,600,000	228,600,000	231,700,000	226,100,000	213,200,000	202,400,000	161,700,000	56,300,000
Accident Year 2016 | Casualty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	137,100,000	137,100,000	251,200,000	260,300,000	252,900,000	243,000,000	243,500,000	231,100,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	2,231
Cumulative paid claims and allocated claim adjustment expense, Net	$ 137,100,000	137,100,000	137,100,000	125,600,000	95,600,000	63,600,000	33,300,000	9,100,000
Accident Year 2016 | Specialty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	188,200,000	188,200,000	211,600,000	224,000,000	228,900,000	236,100,000	238,300,000	237,200,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	937
Cumulative paid claims and allocated claim adjustment expense, Net	$ 188,200,000	188,200,000	194,300,000	192,900,000	182,700,000	164,700,000	150,300,000	$ 58,400,000
Accident Year 2017 | Property Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	261,600,000	261,600,000	273,000,000	251,300,000	250,000,000	256,800,000	293,800,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	9,805
Cumulative paid claims and allocated claim adjustment expense, Net	$ 261,600,000	261,600,000	234,800,000	240,900,000	220,000,000	187,300,000	96,000,000
Accident Year 2017 | Casualty Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	61,400,000	61,400,000	215,900,000	194,700,000	176,700,000	172,900,000	179,400,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	5,470
Cumulative paid claims and allocated claim adjustment expense, Net	$ 61,400,000	61,400,000	89,400,000	95,800,000	52,200,000	22,700,000	3,500,000
Accident Year 2017 | Marine, Aviation and Energy Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	141,400,000	141,400,000	225,600,000	214,200,000	206,700,000	200,200,000	209,600,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	6,078
Cumulative paid claims and allocated claim adjustment expense, Net	$ 141,400,000	141,400,000	149,800,000	168,200,000	140,100,000	97,400,000	40,100,000
Accident Year 2017 | Financial and Professional Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	136,000,000	136,000,000	209,100,000	187,100,000	186,500,000	181,600,000	205,500,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	1,751
Cumulative paid claims and allocated claim adjustment expense, Net	$ 136,000,000	136,000,000	134,800,000	116,800,000	83,200,000	51,200,000	27,100,000
Accident Year 2017 | Property Catastrophe and Other Property Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	431,800,000	431,800,000	573,600,000	501,800,000	513,100,000	531,400,000	552,300,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	1,958
Cumulative paid claims and allocated claim adjustment expense, Net	$ 431,800,000	431,800,000	411,700,000	438,400,000	414,500,000	356,100,000	123,200,000
Accident Year 2017 | Casualty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	110,800,000	110,800,000	261,600,000	250,400,000	251,000,000	240,400,000	242,700,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	2,284
Cumulative paid claims and allocated claim adjustment expense, Net	$ 110,800,000	110,800,000	110,700,000	97,000,000	58,800,000	30,400,000	8,800,000
Accident Year 2017 | Specialty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	305,900,000	305,900,000	356,400,000	363,000,000	374,100,000	390,400,000	377,400,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	1,336
Cumulative paid claims and allocated claim adjustment expense, Net	$ 305,900,000	305,900,000	306,400,000	305,200,000	270,000,000	238,200,000	$ 94,500,000
Accident Year 2018 | Property Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	180,200,000	180,200,000	196,300,000	186,800,000	203,000,000	200,700,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	8,395
Cumulative paid claims and allocated claim adjustment expense, Net	$ 180,200,000	180,200,000	174,100,000	180,400,000	158,100,000	61,400,000
Accident Year 2018 | Casualty Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	43,700,000	43,700,000	164,700,000	134,900,000	124,500,000	121,500,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	5,455
Cumulative paid claims and allocated claim adjustment expense, Net	$ 43,700,000	43,700,000	58,100,000	42,700,000	27,700,000	3,100,000
Accident Year 2018 | Marine, Aviation and Energy Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	151,000,000	151,000,000	234,800,000	208,300,000	207,400,000	170,400,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	5,183
Cumulative paid claims and allocated claim adjustment expense, Net	$ 151,000,000	151,000,000	151,000,000	133,000,000	104,700,000	26,700,000
Accident Year 2018 | Financial and Professional Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	111,200,000	111,200,000	161,900,000	153,200,000	171,600,000	155,700,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	4,636
Cumulative paid claims and allocated claim adjustment expense, Net	$ 111,200,000	111,200,000	111,200,000	99,000,000	73,400,000	19,100,000
Accident Year 2018 | Property Catastrophe and Other Property Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	282,100,000	282,100,000	534,700,000	344,100,000	351,900,000	318,200,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	1,777
Cumulative paid claims and allocated claim adjustment expense, Net	$ 282,100,000	282,100,000	279,500,000	286,200,000	282,200,000	122,400,000
Accident Year 2018 | Casualty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	92,700,000	92,700,000	254,300,000	264,100,000	256,500,000	227,200,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	2,149
Cumulative paid claims and allocated claim adjustment expense, Net	$ 92,700,000	92,700,000	92,700,000	73,300,000	33,400,000	7,100,000
Accident Year 2018 | Specialty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	324,300,000	324,300,000	416,200,000	391,700,000	393,300,000	393,800,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	1,418
Cumulative paid claims and allocated claim adjustment expense, Net	$ 324,300,000	324,300,000	324,700,000	313,000,000	279,600,000	$ 27,100,000
Accident Year 2019 | Property Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	109,000,000	109,000,000	102,800,000	129,200,000	125,300,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	6,944
Cumulative paid claims and allocated claim adjustment expense, Net	$ 109,000,000	109,000,000	98,400,000	90,500,000	48,700,000
Accident Year 2019 | Casualty Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	48,600,000	48,600,000	153,500,000	146,500,000	124,100,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	5,156
Cumulative paid claims and allocated claim adjustment expense, Net	$ 48,600,000	48,600,000	64,100,000	17,600,000	6,300,000
Accident Year 2019 | Marine, Aviation and Energy Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	102,400,000	102,400,000	123,500,000	153,000,000	145,600,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	3,691
Cumulative paid claims and allocated claim adjustment expense, Net	$ 102,400,000	102,400,000	89,600,000	72,500,000	33,500,000
Accident Year 2019 | Financial and Professional Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	132,700,000	132,700,000	238,900,000	261,100,000	248,200,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	23,826
Cumulative paid claims and allocated claim adjustment expense, Net	$ 132,700,000	132,700,000	121,100,000	86,600,000	27,100,000
Accident Year 2019 | Property Catastrophe and Other Property Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	162,700,000	162,700,000	332,400,000	241,500,000	228,000,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	1,401
Cumulative paid claims and allocated claim adjustment expense, Net	$ 162,700,000	162,700,000	167,500,000	140,800,000	28,100,000
Accident Year 2019 | Casualty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	52,900,000	52,900,000	244,300,000	254,000,000	233,500,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	1,790
Cumulative paid claims and allocated claim adjustment expense, Net	$ 52,900,000	52,900,000	52,500,000	36,400,000	9,200,000
Accident Year 2019 | Specialty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	401,100,000	401,100,000	398,700,000	495,900,000	472,600,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Number of Reported Claims | Integer	1,543
Cumulative paid claims and allocated claim adjustment expense, Net	$ 401,100,000	401,100,000	399,200,000	381,000,000	$ 273,200,000
Accident Year 2020 | Property Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	213,500,000	208,000,000	198,300,000	203,200,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 25,900,000
Number of Reported Claims | Integer	7,689
Cumulative paid claims and allocated claim adjustment expense, Net	$ 169,600,000	151,800,000	124,100,000	61,000,000
Accident Year 2020 | Casualty Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	146,600,000	140,400,000	141,600,000	132,900,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 50,200,000
Number of Reported Claims | Integer	3,753
Cumulative paid claims and allocated claim adjustment expense, Net	$ 61,600,000	36,300,000	9,300,000	0
Accident Year 2020 | Marine, Aviation and Energy Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	126,600,000	125,700,000	111,200,000	110,200,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 9,600,000
Number of Reported Claims | Integer	3,847
Cumulative paid claims and allocated claim adjustment expense, Net	$ 101,200,000	88,700,000	66,500,000	28,500,000
Accident Year 2020 | Financial and Professional Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	352,300,000	338,200,000	347,900,000	348,100,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 87,400,000
Number of Reported Claims | Integer	106,054
Cumulative paid claims and allocated claim adjustment expense, Net	$ 226,200,000	174,700,000	121,200,000	47,600,000
Accident Year 2020 | Property Catastrophe and Other Property Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	362,600,000	350,700,000	403,700,000	317,600,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ (5,500,000)
Number of Reported Claims | Integer	1,418
Cumulative paid claims and allocated claim adjustment expense, Net	$ 312,300,000	237,300,000	165,700,000	41,900,000
Accident Year 2020 | Casualty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	181,200,000	198,800,000	234,700,000	253,600,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 62,000,000
Number of Reported Claims | Integer	1,389
Cumulative paid claims and allocated claim adjustment expense, Net	$ 71,900,000	44,300,000	27,800,000	9,100,000
Accident Year 2020 | Specialty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	379,500,000	375,400,000	601,300,000	414,100,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 36,700,000
Number of Reported Claims | Integer	1,494
Cumulative paid claims and allocated claim adjustment expense, Net	$ 311,600,000	290,900,000	270,100,000	$ 213,000,000
Accident Year 2021 | Property Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	195,500,000	202,000,000	207,400,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 16,100,000
Number of Reported Claims | Integer	6,810
Cumulative paid claims and allocated claim adjustment expense, Net	$ 151,200,000	119,700,000	58,700,000
Accident Year 2021 | Casualty Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	197,800,000	188,600,000	173,500,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 99,400,000
Number of Reported Claims | Integer	3,343
Cumulative paid claims and allocated claim adjustment expense, Net	$ 53,800,000	23,500,000	3,100,000
Accident Year 2021 | Marine, Aviation and Energy Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	95,700,000	96,200,000	93,100,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 10,100,000
Number of Reported Claims | Integer	4,787
Cumulative paid claims and allocated claim adjustment expense, Net	$ 64,700,000	52,300,000	23,500,000
Accident Year 2021 | Financial and Professional Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	296,200,000	304,800,000	286,200,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 128,400,000
Number of Reported Claims | Integer	34,929
Cumulative paid claims and allocated claim adjustment expense, Net	$ 131,700,000	90,400,000	43,200,000
Accident Year 2021 | Property Catastrophe and Other Property Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	498,900,000	479,300,000	652,900,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 30,700,000
Number of Reported Claims | Integer	1,498
Cumulative paid claims and allocated claim adjustment expense, Net	$ 364,300,000	235,700,000	75,000,000
Accident Year 2021 | Casualty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	205,700,000	216,300,000	206,400,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 93,300,000
Number of Reported Claims | Integer	1,325
Cumulative paid claims and allocated claim adjustment expense, Net	$ 64,000,000	37,300,000	7,800,000
Accident Year 2021 | Specialty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	142,100,000	152,500,000	157,400,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 41,100,000
Number of Reported Claims | Integer	1,356
Cumulative paid claims and allocated claim adjustment expense, Net	$ 76,100,000	53,400,000	$ 28,300,000
Accident Year 2022 | Property Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	174,200,000	167,100,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 22,300,000
Number of Reported Claims	5,702
Cumulative paid claims and allocated claim adjustment expense, Net	$ 113,700,000	41,100,000
Accident Year 2022 | Casualty Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	215,100,000	204,700,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 148,300,000
Number of Reported Claims	3,050
Cumulative paid claims and allocated claim adjustment expense, Net	$ 5,600,000	8,900,000
Accident Year 2022 | Marine, Aviation and Energy Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	106,100,000	108,000,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 31,200,000
Number of Reported Claims	5,428
Cumulative paid claims and allocated claim adjustment expense, Net	$ 57,600,000	24,900,000
Accident Year 2022 | Financial and Professional Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	302,400,000	317,000,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 184,600,000
Number of Reported Claims	3,422
Cumulative paid claims and allocated claim adjustment expense, Net	$ 75,500,000	17,800,000
Accident Year 2022 | Property Catastrophe and Other Property Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	392,500,000	393,700,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 48,900,000
Number of Reported Claims | Integer	1,404
Cumulative paid claims and allocated claim adjustment expense, Net	$ 200,800,000	65,200,000
Accident Year 2022 | Casualty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	250,600,000	249,500,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 178,300,000
Number of Reported Claims | Integer	1,256
Cumulative paid claims and allocated claim adjustment expense, Net	$ 31,100,000	9,400,000
Accident Year 2022 | Specialty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	194,300,000	194,700,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 108,900,000
Number of Reported Claims | Integer	1,319
Cumulative paid claims and allocated claim adjustment expense, Net	$ 53,800,000	$ 26,000,000
Short-Duration Insurance Contract, Accident Year 2023 | Property Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	152,100,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 81,700,000
Number of Reported Claims	3,297
Cumulative paid claims and allocated claim adjustment expense, Net	$ 30,000,000
Short-Duration Insurance Contract, Accident Year 2023 | Casualty Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	224,900,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 173,000,000
Number of Reported Claims	2,465
Cumulative paid claims and allocated claim adjustment expense, Net	$ 26,900,000
Short-Duration Insurance Contract, Accident Year 2023 | Marine, Aviation and Energy Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	117,400,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 72,600,000
Number of Reported Claims	2,703
Cumulative paid claims and allocated claim adjustment expense, Net	$ 27,800,000
Short-Duration Insurance Contract, Accident Year 2023 | Financial and Professional Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	341,600,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 277,000,000
Number of Reported Claims	3,137
Cumulative paid claims and allocated claim adjustment expense, Net	$ 21,500,000
Short-Duration Insurance Contract, Accident Year 2023 | Property Catastrophe and Other Property Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	230,800,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 117,700,000
Number of Reported Claims | Integer	753
Cumulative paid claims and allocated claim adjustment expense, Net	$ 45,100,000
Short-Duration Insurance Contract, Accident Year 2023 | Casualty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	271,600,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 244,900,000
Number of Reported Claims | Integer	779
Cumulative paid claims and allocated claim adjustment expense, Net	$ 8,500,000
Short-Duration Insurance Contract, Accident Year 2023 | Specialty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	142,500,000
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 100,400,000
Number of Reported Claims | Integer	952
Cumulative paid claims and allocated claim adjustment expense, Net	$ 22,600,000

Reserves for Losses and Adjus_7

Reserves for Losses and Adjustment Expenses Short-duration insurance contracts - Reconciliation of Claims Development (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for claims and claim adjustment expenses, net of reinsurance	$ 3,629.6	$ 3,151.6
Reinsurance recoverable on unpaid losses	4,577.8	4,897.7
Unallocated claims incurred	47.9	41.4
Carbon syndicate reserves	16.7	5.2
Other	0.4	1.6
Liability for unpaid claims and claim adjustment expense, aggregate reconciling items	(396.8)	(338.4)
Losses and loss adjustment expenses	7,810.6	7,710.9	$ 7,611.8	$ 7,165.3
Property Insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for claims and claim adjustment expenses, net of reinsurance	270.8	259
Casualty Insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for claims and claim adjustment expenses, net of reinsurance	636.5	457.3
Marine, Aviation and Energy Insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for claims and claim adjustment expenses, net of reinsurance	194.5	163.7
Financial and Professional Insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for claims and claim adjustment expenses, net of reinsurance	837.6	671.9
Insurance lines
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for claims and claim adjustment expenses, net of reinsurance	1,939.4	1,551.9
Reinsurance recoverable on unpaid losses	2,821.6	2,907.8
Property Catastrophe and Other Property Reinsurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for claims and claim adjustment expenses, net of reinsurance	562.3	677.9
Casualty Reinsurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for claims and claim adjustment expenses, net of reinsurance	733.6	571.6
Specialty Reinsurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for claims and claim adjustment expenses, net of reinsurance	394.3	350.2
Reinsurance lines
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for claims and claim adjustment expenses, net of reinsurance	1,690.2	1,599.7
Reinsurance recoverable on unpaid losses	1,756.2	1,989.9
Insurance lines other than short-duration
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Other reinsurance balances recoverable	(489.1)	(429.3)
Losses and loss adjustment expenses	$ 27.3	$ 42.7

Reserves for Losses and Adjus_8

Reserves for Losses and Adjustment Expenses Short-duration Contracts - Historical Claims Duration (Details)	Dec. 31, 2023
Property Insurance
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year One	30.60%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Two	39%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Three	13.70%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Four	6.30%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Five	2.70%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Six	2.40%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Seven	(0.20%)
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Eight	(0.30%)
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Nine	(0.70%)
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance, Year Ten	0%
Casualty Insurance
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year One	4.10%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Two	18.70%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Three	33.10%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Four	24.20%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Five	2.70%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Six	1%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Seven	(5.10%)
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Eight	(0.40%)
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Nine	(0.10%)
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance, Year Ten	0%
Marine, Aviation and Energy Insurance
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year One	22.50%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Two	33.30%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Three	21.50%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Four	11.40%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Five	3.20%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Six	3.10%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Seven	(2.80%)
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Eight	(0.70%)
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Nine	(0.40%)
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance, Year Ten	0%
Financial and Professional Insurance
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year One	12.10%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Two	28.60%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Three	20.70%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Four	16%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Five	5.30%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Six	6%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Seven	(4.70%)
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Eight	(1.40%)
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Nine	(0.30%)
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance, Year Ten	0%
Property Catastrophe and Other Property Reinsurance
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year One	22.60%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Two	45.50%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Three	16.70%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Four	5.80%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Five	2.30%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Six	1.50%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Seven	(0.20%)
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Eight	(0.70%)
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Nine	(0.10%)
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance, Year Ten	0%
Casualty Reinsurance
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year One	6%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Two	19%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Three	22.40%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Four	19.30%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Five	12.50%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Six	8.10%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Seven	(5.20%)
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Eight	(1.70%)
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Nine	0%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance, Year Ten	0%
Specialty Reinsurance
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year One	27.30%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Two	34.90%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Three	14.30%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Four	7.40%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Five	3.70%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Six	1.10%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Seven	(0.50%)
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Eight	0%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Nine	0%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance, Year Ten	0%

Income Taxes - Summary of Total

Income Taxes - Summary of Total Income Tax (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Tax Disclosure [Abstract]
Income tax (benefit)/expense allocated to net income	$ (132.1)	$ (78.1)	$ 5.3
Income tax (benefit)/expense allocated to other comprehensive income	20.6	(23.9)	0.3
Total income tax (benefit)/expense	$ (111.5)	$ (102)	$ 5.6

Income Taxes - Income_(Loss) Be

Income Taxes - Income/(Loss) Before Tax and Income Tax Expense/(Benefit) Attributable to that Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Tax Examination [Line Items]
(Loss) before tax, Non-U.S.	$ 16.8	$ (20.9)	$ (67.9)
(Loss) before tax, U.S.	236.3	34.8	5
Income (loss) from operations before income taxes	402.6	(27)	35.1
Current tax (benefit)/expense, Non-U.S.	7.9	4.7	2.7
Current tax (benefit)/expense, U.S.	52.4	14.8	5.8
Current tax (benefit)/expense, Total	65.6	26.5	8.5
Deferred tax (benefit), Non-U.S.	0.3	(1.7)	(2.9)
Deferred tax (benefit), U.S.	3	(102.9)	0
Deferred tax, Total	(197.7)	(104.6)	(3.2)
Total tax expense/(benefit), Non-U.S.	8.2	3	(0.2)
Total tax expense/(benefit), U.S.	55.4	(88.1)	5.8
Total tax expense/(benefit)	(132.1)	(78.1)	5.3
Base erosion and anti-abuse (BEAT) expense	0.9	2.3	6.1
Bermuda
Income Tax Examination [Line Items]
(Loss) before tax, Non-U.S.	148.8	(103.3)	22.9
Current tax (benefit)/expense, Non-U.S.	0	0	0
Deferred tax (benefit), Non-U.S.	(201.1)	0	0
Total tax expense/(benefit), Non-U.S.	(201.1)	0	0
Net deferred tax asset, Bermuda	156.6
Opening tax loss adjustment	$ 44.5
Deferred tax asset, utilization period	10 years
U.K.
Income Tax Examination [Line Items]
(Loss) before tax, Non-U.S.	$ 0.7	62.4	75.1
Current tax (benefit)/expense, Non-U.S.	5.3	7	0
Deferred tax (benefit), Non-U.S.	0.1	0	(0.3)
Total tax expense/(benefit), Non-U.S.	$ 5.4	$ 7	$ (0.3)

Income Taxes - Income Tax Recon

Income Taxes - Income Tax Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Operating Loss Carryforwards [Line Items]
Income tax benefit at statutory tax rate of zero percent	$ 0	$ 0	$ 0
Overseas statutory tax rates differential	56.3	16.8	(0.9)
Base erosion and anti-abuse (BEAT) expense	0.9	2.3	6.1
Prior year adjustments	6.9	(2.9)	0.5
Introduction of Bermuda corporate income tax	(201.1)	0	0
Change in valuation allowance (2)	4	(98.9)	9.6
Impact of unrecognized tax benefits	0	0	0
Australian non-resident withholding tax	0	1.5	0.6
Foreign exchange	(1.3)	(0.3)	(1.5)
Non-deductible expenses	2.5	2.4	2.4
Impact of changes in statutory tax rates (4)	(0.3)	(5.7)	(11.5)
Effective Income Tax Rate Reconciliation, Tax Effect of OCI, Amount	0	6.7	0
Total tax expense/(benefit)	$ (132.1)	$ (78.1)	$ 5.3

Income Taxes - Tax Effects of D

Income Taxes - Tax Effects of Deferred Tax Assets and Deferred Tax Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Income Tax Disclosure [Abstract]
Operating loss carryforwards	$ 217.2	$ 167.6
Capital loss carryforwards	9.7	6.7
Insurance reserves: Losses and loss adjustment expenses	104.1	28.3
Unrealized losses on investments	8.9	20.6
Accrued expenses	13.4	6.1
Foreign tax credit carryforwards	19	19.8
Unearned premiums	35	36
Intangible assets	82.9	0.7
Office properties and equipment	16.5	14.2
Operating lease liability	15.6	18.5
Other temporary differences	7.6	3.7
Total deferred tax assets	529.9	322.2
Less valuation allowance	(172.7)	(145.7)
Deferred tax assets, net of valuation allowance	357.2	176.5
Deferred acquisition costs	(32.4)	(37)
Operating lease assets	(10.4)	(13.7)
Deferred Tax Liabilities, GAAP differences	0.1	0.2
Other temporary differences	(3.3)	(6.4)
Total deferred tax (liabilities)	(46.2)	(57.3)
Deferred Tax Assets, Net	$ 311	$ 119.2

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2022
Income Taxes [Line Items]
United States corporate tax at a rate	21%
Capital loss carryforwards	$ 9.7			$ 6.7
Less valuation allowance	172.7			145.7
Deferred Tax Assets, Gross	529.9			322.2
Internal Revenue Service (IRS)
Income Taxes [Line Items]
Net operating loss carryforwards	354.9			376.5
Operating loss carryforwards, remaining	354.9
Capital loss carryforwards	46.1
Less valuation allowance	24.5			18
Internal Revenue Service (IRS) | Aspen U.K.
Income Taxes [Line Items]
Capital loss carryforwards	30.3
Operating loss carryforwards, subject to s382 limitation	39.2
Internal Revenue Service (IRS) | Aspen U.K. | Subsequent Event
Income Taxes [Line Items]
Operating loss carryforwards, subject to s382 limitation			$ 23.1
Internal Revenue Service (IRS) | U.S. operating subsidiaries
Income Taxes [Line Items]
Net operating loss carryforwards	270.2
Capital loss carryforwards	15.8
Internal Revenue Service (IRS) | U.S. operating subsidiaries | Subsequent Event
Income Taxes [Line Items]
Operating loss carryforwards, subject to s382 limitation		$ 20.8
Internal Revenue Service (IRS) | Aspen U.K.'s U.S. branch
Income Taxes [Line Items]
Net operating loss carryforwards	84.7
Operating loss carryforwards, subject to s382 limitation	$ 6.5
Internal Revenue Service (IRS) | Minimum
Income Taxes [Line Items]
Operating loss carryforwards, date of expiration	2032
Internal Revenue Service (IRS) | Maximum
Income Taxes [Line Items]
Operating loss carryforwards, date of expiration	2041
U.K.
Income Taxes [Line Items]
U. K. corporate tax rate	23.50%
Net operating loss carryforwards	$ 248.1			280.5
Operating loss carryforwards, remaining	19			19.8
Less valuation allowance	131			106.7
U.K. | Lloyd's
Income Taxes [Line Items]
Deferred tax losses, operating loss carryforward	64.5			(19.9)
U.K. | Other juridictions
Income Taxes [Line Items]
Deferred tax losses, operating loss carryforward	$ (97.8)			$ (118.8)
Bermuda
Income Taxes [Line Items]
Bermuda tax rate	0%

Capital Structure - Summary of

Capital Structure - Summary of Authorized and Issued Share Capital (Details) - USD ($) $ / shares in Units, $ in Thousands				12 Months Ended
	Aug. 13, 2019	Sep. 20, 2016	May 02, 2013	Dec. 31, 2023	Dec. 31, 2022
Authorized share capital:
Number of ordinary shares				70,000,000	70,000,000
Number of preference shares				30,000,000	30,000,000
Ordinary Shares, authorized				$ 700	$ 700
Preference Shares, authorized				45	45
Total authorized share capital				$ 745	$ 745
Issued share capital:
Ordinary shares, issued				60,395,839	60,395,839
Ordinary shares, value				$ 604	$ 604
Total issued share capital				$ 636	$ 636
Ordinary shares, par value				$ 0.01	$ 0.01
Preference shares, par value				$ 0.0015144558
5.950% Preference Shares (AHL PRC)
Issued share capital:
Preference shares, issued			11,000,000	11,000,000	11,000,000
Preference shares, value				$ 17	$ 17
Preferred Stock, Dividend Rate, Percentage			5.95%	5.95%	5.95%
Preference shares, par value				$ 0.0015144558	$ 0.0015144558
Redemption price per share			$ 25
5.625% Preference Shares (AHL PRD)
Issued share capital:
Preference shares, issued		10,000,000		10,000,000	10,000,000
Preference shares, value				$ 15	$ 15
Preferred Stock, Dividend Rate, Percentage		5.625%		5.625%	5.625%
Preference shares, par value				$ 0.0015144558	$ 0.0015144558
Redemption price per share		$ 25		$ 25	$ 25
5.625% Preference Shares, rep by Dep Shares (AHL PRE)
Issued share capital:
Preference shares, issued	10,000,000			10,000,000	10,000,000
Preference shares, value				$ 0	$ 0
Preferred Stock, Dividend Rate, Percentage	5.625%			5.625%	5.625%
Preference shares, par value				$ 0.0015144558	$ 0.0015144558
Redemption price per share	$ 25			$ 25	$ 25

Capital Structure - Summary o_2

Capital Structure - Summary of Authorized and Issued Share Capital (Details)(Phantom) - $ / shares	Dec. 31, 2023	Dec. 31, 2022	Aug. 13, 2019	Sep. 20, 2016	May 02, 2013
Class of Stock [Line Items]
Ordinary shares, par value	$ 0.01	$ 0.01
Preference shares, par value	0.0015144558
5.950% Preference Shares (AHL PRC)
Class of Stock [Line Items]
Preference shares, par value	0.0015144558	0.0015144558
Redemption price per share					$ 25
5.625% Preference Shares (AHL PRD)
Class of Stock [Line Items]
Preference shares, par value	0.0015144558	0.0015144558
Redemption price per share	25	25		$ 25
5.625% Preference Shares, rep by Dep Shares (AHL PRE)
Class of Stock [Line Items]
Preference shares, par value	0.0015144558	0.0015144558
Redemption price per share	$ 25	$ 25	$ 25

Capital Structure - Additional

Capital Structure - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands				12 Months Ended
	Aug. 13, 2019	Sep. 20, 2016	May 02, 2013	Dec. 31, 2023	Dec. 31, 2022
Class of Stock [Line Items]
Additional paid in capital				$ 761,200	$ 761,200
Aggregate liquidation preferences				775,000	775,000
Preferred stock issuance cost				21,500	21,500
Authorized share capital				$ 745	$ 745
Number of ordinary shares				70,000,000	70,000,000
Ordinary shares, par value				$ 0.01	$ 0.01
Number of preference shares				30,000,000	30,000,000
Preference shares, par value				$ 0.0015144558
Ordinary shares, issued				60,395,839	60,395,839
Preference shares liquidation preference, value	$ 250,000
5.950% Preference Shares (AHL PRC)
Class of Stock [Line Items]
Preferred stock issuance cost			$ 4,400
Preference shares, par value				$ 0.0015144558	$ 0.0015144558
Preference shares, issued			11,000,000	11,000,000	11,000,000
Preference shares, rate			5.95%	5.95%	5.95%
Redemption price per share			$ 25
Proceeds or share issuance			$ 270,600
Preference shares liquidation preference, value			$ 275,000
Fixed for floating rate			0.0959343
5.625% Preference Shares (AHL PRD)
Class of Stock [Line Items]
Preferred stock issuance cost		$ 8,700
Preference shares, par value				$ 0.0015144558	$ 0.0015144558
Preference shares, issued		10,000,000		10,000,000	10,000,000
Preference shares, rate		5.625%		5.625%	5.625%
Redemption price per share		$ 25		$ 25	$ 25
Proceeds or share issuance		$ 241,300
Preference shares liquidation preference, value		$ 250,000
5.625% Preference Shares, rep by Dep Shares (AHL PRE)
Class of Stock [Line Items]
Preferred stock issuance cost	$ 8,400
Preference shares, par value				$ 0.0015144558	$ 0.0015144558
Preference shares, issued	10,000,000			10,000,000	10,000,000
Preference shares, rate	5.625%			5.625%	5.625%
Redemption price per share	$ 25			$ 25	$ 25
Additional paid-in capital | Preference shares
Class of Stock [Line Items]
Proceeds or share issuance	$ 241,600

Earnings Per Share (Details)

Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Earnings Per Share [Abstract]
Net (loss)/income attributable to Aspen Insurance Holdings Limited’s ordinary shareholders	$ 534.7	$ 51.1	$ 29.8
Preference share dividends	(49.9)	(44.6)	(44.5)
Net income/(loss) available to Aspen Insurance Holdings Limited’s ordinary shareholders	$ 484.8	$ 6.5	$ (14.7)
Basic and diluted weighted average ordinary shares outstanding	60,395,839	60,395,839	60,395,839
Earnings Per Share, Diluted	$ 8.03	$ 0.11	$ (0.24)
Earnings Per Share, Basic	$ 8.03	$ 0.11	$ (0.24)

Statutory Requirements and Di_3

Statutory Requirements and Dividends Restrictions - Summary of Statutory Requirements and Dividends Restrictions (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
U.S.
Required statutory capital and surplus	$ 488.9	$ 513.9
Actual statutory capital and surplus	1,063.1	838.6
Bermuda
Required statutory capital and surplus	601.1	536.7
Actual statutory capital and surplus	1,685.2	1,426.6
U.K.
Required statutory capital and surplus	257.2	771.8
Actual statutory capital and surplus	$ 734.7	$ 802.5

Statutory Requirements and Di_4

Statutory Requirements and Dividends Restrictions - Additional Information (Details) - 12 months ended Dec. 31, 2023 £ in Millions, $ in Millions	USD ($)	GBP (£)
AUL
Statutory Accounting Practices [Line Items]
Syndicate to maintain funds at Lloyd	$ 989.9
Total funds held by AUL	1,101
U.K.
Statutory Accounting Practices [Line Items]
Dividend payment made without regulatory approval	(599.3)	£ (78)
Capital contributions reserves	655
Bermuda
Statutory Accounting Practices [Line Items]
Dividend payment made without regulatory approval	$ (303.9)
Statutory capital and surplus, percent	25%
Statutory capital and surplus, percent reduction requiring approval	15%
Percent warning level of amount of enhanced capital required from statutory capital and surplus	120%
Bermuda | AUL
Statutory Accounting Practices [Line Items]
Total funds held by AUL	$ 515.4

Dividends - Summary of Declared

Dividends - Summary of Declared Dividends (Details) - USD ($)					3 Months Ended				12 Months Ended
	Dec. 28, 2023	Aug. 13, 2019	Sep. 20, 2016	May 02, 2013	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Dividends Payable [Line Items]
Dividends on preference shares					$ 13,773,500	$ 13,847,200	$ 11,122,500	$ 11,122,500	$ 49,865,700
Dividends Payable, Date Declared					Nov. 30, 2023	Sep. 01, 2023	Jun. 01, 2023	Mar. 01, 2023
Payable Date	Dec. 28, 2023					Oct. 01, 2023	Jul. 01, 2023	Apr. 01, 2023
Dividends paid on ordinary shares									40,300,000	$ 40,000,000	$ 0
5.950% Preference Shares (AHL PRC)
Dividends Payable [Line Items]
Dividends on preference shares					$ 6,741,900	$ 6,815,600	$ 4,090,900	$ 4,090,900	$ 21,739,300
Preferred Stock, Dividend Rate, Percentage				5.95%					5.95%	5.95%
5.625% Preference Shares, rep by Dep Shares (AHL PRE)
Dividends Payable [Line Items]
Dividends on preference shares					$ 3,515,600	3,515,600	3,515,600	3,515,600	$ 14,062,400
Depositary share dividend					$ 0.35156				$ 0.35156
Preferred Stock, Dividend Rate, Percentage		5.625%							5.625%	5.625%
5.625% Preference Shares (AHL PRD)
Dividends Payable [Line Items]
Dividends on preference shares					$ 3,516,000	$ 3,516,000	$ 3,516,000	$ 3,516,000	$ 14,064,000
Preferred Stock, Dividend Rate, Percentage			5.625%						5.625%	5.625%

Retirement Plans (Details)

Retirement Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Retirement Benefits [Abstract]
Total contributions by the Company to the retirement plan	$ 14.5	$ 12.5	$ 13.5

Share-Based Payments and Long_2

Share-Based Payments and Long-term Incentive Plan - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment and LTIP expense	$ 5.5	$ 0.6
Accrued expenses and other payables (4)	$ 214.4	201.8
Weighted Average Exercise Price	$ 0.001
Number of options granted	10,000
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share-based compensation expense	$ 5.5	0.6
Long-term incentive plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Accrued expenses and other payables (4)	7.6	$ 1.6
Management equity plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share-based compensation expense	7.5
Unrecognized Share Based Compensation Expense	$ 27.5

Intangible Assets - Information

Intangible Assets - Information (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Finite-Lived Intangible Assets [Line Items]
Intangible assets and goodwill	$ 21.7	$ 21.8
Goodwill	3.9
Aspen trademark
Finite-Lived Intangible Assets [Line Items]
Value of the asset	1.1
Insurance Licenses
Finite-Lived Intangible Assets [Line Items]
Value of the asset	$ 16.7

Operating Leases - Schedule of

Operating Leases - Schedule of Operating Lease Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Leases [Abstract]
Amortization of right-of-use operating lease assets	$ 10.7	$ 10.1	$ 12
Interest on operating lease liability	4.5	5.4	5.5
Operating Lease, Expense	$ 15.2	$ 15.5	$ 17.5

Operating Leases - Schedule o_2

Operating Leases - Schedule of Operating Lease Liability, Maturity (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Leases [Abstract]
Liability, Payments, Due prior year	$ 0	$ 15.4
Liability, Payments, Next twelve months	15.4	15.2
Liability, Payments, Due year two	15.1	14.9
Liability, Payments, Due year three	14.3	14.1
Liability, Payments, Due year four	12.8	12.6
Liability, Payments, Due year five	12.7	12.4
Liability, Payments, Due after year five	32.4	34.8
Total minimum lease payments	102.7	119.4
Less imputed interest	(16.6)	(23.9)
Operating lease liabilities	$ 86.1	$ 95.5

Operating Leases - Schedule o_3

Operating Leases - Schedule of Operating Leases Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Leases [Abstract]
Operating lease liabilities	$ (15.5)	$ (15.5)	$ (17.5)
Asset obtained in exchange for operating lease liability	0.2	1.9	23.9
Reduction of right-of use assets resulting from reductions to lease obligations	$ 0.1	$ 7	$ 2.3
Operating lease, weighted average remaining lease term (years(	7 years 3 months 18 days	8 years 1 month 6 days	8 years 9 months 18 days
Operating lease, weighted average discount rate (percent)	5%	5%	5%

Operating Leases - Additional I

Operating Leases - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Leases [Abstract]
Right-of-use operating lease assets	$ 61,600,000	$ 72,800,000
Operating lease liabilities	86,100,000	95,500,000
Impairment of lease assets	$ 0	$ (6,700,000)	$ 400,000

Related Party Transactions - Na

Related Party Transactions - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Related Party Transaction [Line Items]
Plus component of annual investment management fee	0.25
Annual fee cap	0.15
Privately-held Investments	$ 475	$ 533
Other investments, at fair value (2)	209.3	221.3
Net investment income (1)	275.7	188.1	$ 147.5
Accrued expenses and other payables (4)	214.4	201.8
Realized and unrealized investment gains	75.9	5	56.2
Debt Securities, Trading	1,485.7	1,475.5
Other expenses	0	20.1	10.8
Asset Management Arrangement
Related Party Transaction [Line Items]
Net investment income (1)	9.4	4.9	$ 5.8
Accrued expenses and other payables (4)	2.1	4.5
Apollo real estate fund
Related Party Transaction [Line Items]
Realized and unrealized investment gains	0.4	3.1
Beneficial owner
Related Party Transaction [Line Items]
Accounts Payable, Current	1.2	2
Apollo real estate fund | Fair Value, Recurring [Member]
Related Party Transaction [Line Items]
Other investments, at fair value (2)	23.9	25.3
Funds managed by Apollo | Fair Value, Recurring [Member]
Related Party Transaction [Line Items]
Other investments, at fair value (2)	39.8	38
Apollo Notes, SPV
Related Party Transaction [Line Items]
Privately-held investment, losses incurred	5.5	(0.4)
Apollo Notes, SPV | Fair Value, Recurring [Member]
Related Party Transaction [Line Items]
Privately-held Investments	82.2	44.8
CLOs
Related Party Transaction [Line Items]
Net investment income (1)	17.4	0
Other expenses	0.5
CLOs | Fair Value, Recurring [Member]
Related Party Transaction [Line Items]
Debt Securities, Trading	129.8	0
Middle Market Term Loans managed by a subsidiary of Apollo
Related Party Transaction [Line Items]
Net investment income (1)	5.8	0
Other expenses	0.2
Middle Market Term Loans managed by a subsidiary of Apollo | Fair Value, Recurring [Member]
Related Party Transaction [Line Items]
Debt Securities, Trading	$ 45.1	$ 0
Maximum
Related Party Transaction [Line Items]
Percentage of consolidated group's net income due to Apollo Management	1%
Dollar amount option to percentage fee	$ 5

Commitments and Contingencies -

Commitments and Contingencies - Schedule of Company's Restricted Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Fair Value, Separate Account Investment [Line Items]
Pledged Financial Instruments, Not Separately Reported, Securities for Intercompany Reinsurance Agreements	$ 660.8	$ 707.1
Pledged Financial Instruments, Not Separately Reported, Securities for Third-Party Agreements	2,714.4	2,817.7
Pledged Financial Instruments, Not Separately Reported Securities for Letter of Credit Facility	172	471.3
Restricted assets, excluding illiquid assets	3,547.2	3,996.1
Other investments, at fair value (2)	209.3	221.3
Debt Securities, Available for Sale and Held for Trading, and Illiquid Assets, Restricted	$ 3,756.5	$ 4,217.4
Total as percent of investable assets	50.20%	59.40%

Commitments and Contingencies_2

Commitments and Contingencies - Additional Information (Details) £ in Millions, SFr in Millions, $ in Millions, $ in Millions, $ in Millions, $ in Millions	1 Months Ended	12 Months Ended
	Mar. 31, 2021 USD ($)	Dec. 31, 2023 USD ($) Investment	Dec. 31, 2022 USD ($) Investment	Dec. 31, 2021 USD ($)	Dec. 31, 2023 GBP (£) Investment	Dec. 31, 2023 CAD ($) Investment	Dec. 31, 2023 AUD ($) Investment	Dec. 31, 2023 CHF (SFr) Investment	Dec. 31, 2023 SGD ($) Investment	Dec. 31, 2022 GBP (£) Investment	Dec. 31, 2022 CAD ($) Investment	Dec. 31, 2022 AUD ($) Investment	Dec. 31, 2022 CHF (SFr) Investment	Dec. 31, 2022 SGD ($) Investment	Apr. 01, 2022 USD ($)	Sep. 30, 2021 USD ($)	Dec. 23, 2019 USD ($)	Dec. 20, 2017 USD ($)
Restricted Cash And Collateral [Line Items]
Other investments (equity method)		$ 209.3	$ 221.3
Total as percent of investable assets		50.20%	59.40%
Investment in the period		$ 0.4	$ 2
Commitment to invest in private assets		99	377.9	$ 205.1
Investable assets held by the Company		7,500	7,100
Investable assets held by insurance regulators		$ 517.4	489.5
Minimum capital required | £					£ 0.5					£ 0.4
Percentage of reinsurance liabilities		100%			100%	100%	100%	100%	100%
Reinsurance liabilities		$ 823.5	1,166.4
Assets held-in-trust		1,016.9	1,389.5
Regulatory deposits		6.8	6.7
Deposit with states		$ 6.4	$ 6.5
Number of investments | Investment		1	1		1	1	1	1	1	1	1	1	1	1
MVI
Restricted Cash And Collateral [Line Items]
Investment in the period	$ 0.8	$ 0	$ 0.4
Multi-Line Insurer
Restricted Cash And Collateral [Line Items]
Other investments (equity method)																	$ 5
Investment in the period		0.4	1.6
Us Multi Beneficiary Trust Fund
Restricted Cash And Collateral [Line Items]
Minimum trust fund		20
Assets held-in-trust		394.7	509.2
Us Surplus Lines Trust Fund
Restricted Cash And Collateral [Line Items]
Assets held-in-trust		126.6	215.1
Canadian Trust Fund
Restricted Cash And Collateral [Line Items]
Assets held-in-trust		168.4				$ 228.4					$ 185.8
Australian Trust Fund
Restricted Cash And Collateral [Line Items]
Assets held-in-trust		86.9					$ 131					$ 183.9
Swiss Trust Fund
Restricted Cash And Collateral [Line Items]
Assets held-in-trust		11.4						SFr 9.9					SFr 9.4
Singapore Trust Fund
Restricted Cash And Collateral [Line Items]
Assets held-in-trust		$ 144							$ 192.1					$ 174.7
Bermuda
Restricted Cash And Collateral [Line Items]
Percentage of reinsurance liabilities		100%			100%	100%	100%	100%	100%
Reinsurance liabilities		$ 320.6	$ 380.3
Limited Partner
Restricted Cash And Collateral [Line Items]
Restricted assets															$ 10	$ 20		$ 100

Concentration of Credit Risk -

Concentration of Credit Risk - Additional Information (Details) $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Concentration Risk [Line Items]
Reinsurance Recoverable for Paid and Unpaid Claims and Claims Adjustments	$ 4,577.8	$ 4,897.7
Concentration risk	0.568	0.573
Reinsurance Recoverable, Allowance for Credit Loss	$ 3.7	$ 3.7	$ 3.3
Underwriting premiums receivables (net of allowance for expected credit losses of 2022: $25.0 — 2021: $30.2)	1,435.3	1,482.4
Due for settlement	8.7
Premium Receivable, Allowance for Credit Loss	$ 21	$ 25
Allowable holdings of a single issue or issuer, percentage	2%
Maximum
Concentration Risk [Line Items]
Gross written premiums, percentage	15.90%	16.30%
Minimum
Concentration Risk [Line Items]
Gross written premiums, percentage	9.20%	8.20%
Median
Concentration Risk [Line Items]
Gross written premiums, percentage	11.10%	9.70%
A- rating or higher
Concentration Risk [Line Items]
Concentration risk	0.429	0.423
A- rating or lower
Concentration Risk [Line Items]
Concentration risk	0.003	0.004

Reclassifications from Accumu_3

Reclassifications from Accumulated Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income Reclassification (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income (loss) from operations before income taxes	$ 402.6	$ (27)	$ 35.1
Income tax (expense)/benefit	132.1	78.1	(5.3)
General, administrative and corporate expenses	(503.6)	(494.2)	(418)
Net income/(loss)	534.7	51.1	29.8
Net (loss)	534.7	51.1	29.8
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income/(loss)	25.5	70.4	(26.6)
Available for sale securities | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Available for sale securities — gross realized (losses)	(2.2)	(3.9)	(24.8)
Realized losses on sale of securities and cash and cash equivalents	42.4	58.9	4.4
Income (loss) from operations before income taxes	40.2	55	(20.4)
Income tax (expense)/benefit	(6.6)	0	0
Net (loss)	33.6	55	(20.4)
Realized derivatives | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax (expense)/benefit	0	0	0
General, administrative and corporate expenses	(8.1)	15.4	(6.2)
Net income/(loss)	$ (8.1)	$ 15.4	$ (6.2)

Credit Facilities and Long-Te_2

Credit Facilities and Long-Term Debt - Summary of Contractual Obligations Under Long-term Debts (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Line of Credit Facility [Line Items]
Short-term debt	$ 0	$ 299.9
Long-Term Debt, Maturity, Year Two and Three	300
Long-Term Debt, Maturity, Year Four and Five	0
Long-Term Debt, Maturity, after Year Five	0
Debt, Long-Term and Short-Term, Combined Amount	300
Short-Term Debt
Line of Credit Facility [Line Items]
Short-term debt	$ 0

Credit Facilities and Long-Te_3

Credit Facilities and Long-Term Debt - Additional Information (Details) - USD ($)

12 Months Ended

Dec. 01, 2021

Dec. 31, 2023

Dec. 31, 2022

Oct. 24, 2023

Jul. 26, 2023

Feb. 07, 2023

Dec. 29, 2021

Nov. 30, 2021

Nov. 05, 2021

Apr. 01, 2021

Nov. 30, 2020

Nov. 03, 2020

Feb. 11, 2019

Line of Credit Facility [Line Items]

Revolving credit facility, credit agreement maximum limit

$ 300,000,000

Line of credit facility Increasable capacity

$ 100,000,000

Minimum consolidated tangible net worth under credit facility

$ 2,019,600,000

Percentage of consolidated net income

25%

Percentage of aggregate net cash proceeds from the issuance of capital stock

25%

Percentage of consolidated leverage ratio permitted

35%

AUL | Lloyd's

Line of Credit Facility [Line Items]

Letter of Credit to Support Funds at Lloyd's

$ 335,000,000

Funds at Lloyd's Facility Agreement

$ 80,000,000

$ 150,000,000

AUL | Lloyd's | Maximum

Line of Credit Facility [Line Items]

Letter of Credit to Support Funds at Lloyd's

$ 235,000,000

Bermuda | Natixis

Line of Credit Facility [Line Items]

Revolving credit facility, credit agreement maximum limit

$ 100,000,000

Bermuda | HSBC Bank USA, National Association

Line of Credit Facility [Line Items]

Revolving credit facility, credit agreement maximum limit

$ 75,000,000

Aspen Specialty | Federal Home Loan Bank of Boston

Line of Credit Facility [Line Items]

Revolving credit facility, credit agreement maximum limit

$ 250,000,000

Percentage limit of pledged assets permitted to secure debt obligations

15%

AAIC | Federal Home Loan Bank of Boston

Line of Credit Facility [Line Items]

Revolving credit facility, credit agreement maximum limit

$ 174,000,000

Percentage limit of pledged assets permitted to secure debt obligations

10%

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

$ 100,000,000

Secured and unsecured financing facilities

Line of Credit Facility [Line Items]

Long-term line of credit

$ 2,300,000,000

2026 Term Loan

Line of Credit Facility [Line Items]

Loans issued

$ 300,000,000

Interest Expense

$ 3,000,000

$ 0

Credit Losses (Details)

Credit Losses (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Credit Loss [Abstract]
Available-for-sale Investments, Allowance for Credit Loss, Beginning Balance	$ 7.7	$ 2.7
Debt Securities, Available-for-Sale, Allowance for Credit Loss, Not Previously Recorded	0.3	5.4
Available-for-sale investment, allowance for credit loss, increase (decrease)	(3.6)	0.2
Debt Securities, Available-for-Sale, Allowance for Credit Loss, Securities Sold	(1.5)	(0.6)
Available-for-sale Investments, Allowance for Credit Loss, Current	2.9	7.7	$ 2.7
Reinsurance Recoverable, Allowance for Credit Loss, Beginning Balance	3.7	3.3
Reinsurance Recoverable, Allowance for Credit Loss, Period Increase (Decrease)	0	0.4	0.5
Reinsurance Recoverable, Allowance for Credit Loss, Ending Balance	3.7	3.7	3.3
Accounts Receivable, Allowance for Credit Loss, Beginning Balance	25	30.2
Accounts Receivable, Allowance for Credit Loss, Period Increase (Decrease)	(4)	(5.2)
Accounts Receivable, Allowance for Credit Loss, Ending Balance	$ 21	$ 25	$ 30.2

Subsequent Events - Narrative (

Subsequent Events - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions

3 Months Ended

12 Months Ended

Apr. 21, 2024

Apr. 01, 2024

Mar. 15, 2024

Dec. 28, 2023

Aug. 13, 2019

Sep. 20, 2016

May 02, 2013

Dec. 31, 2023

Sep. 30, 2023

Jun. 30, 2023

Mar. 31, 2023

Dec. 31, 2023

Dec. 31, 2022

Dec. 31, 2021

Mar. 01, 2024

Subsequent Event [Line Items]

Payable Date

Dec. 28, 2023

Oct. 01, 2023

Jul. 01, 2023

Apr. 01, 2023

Dividends Payable, Date Declared

Nov. 30, 2023

Sep. 01, 2023

Jun. 01, 2023

Mar. 01, 2023

Dividends paid on ordinary shares

$ 40.3

$ 40

$ 0

5.950% Preference Shares (AHL PRC)

Subsequent Event [Line Items]

Preferred Stock, Dividend Rate, Percentage

5.95%

5.95%

5.95%

5.625% Preference Shares (AHL PRD)

Subsequent Event [Line Items]

Preferred Stock, Dividend Rate, Percentage

5.625%

5.625%

5.625%

5.625% Preference Shares (AHL PRE)

Subsequent Event [Line Items]

Preferred Stock, Dividend Rate, Percentage

5.625%

5.625%

5.625%

Depositary share dividend

$ 0.35156

$ 0.35156

Subsequent Event

Subsequent Event [Line Items]

Dividends paid on ordinary shares

$ 25

Subsequent Event | 5.950% Preference Shares (AHL PRC)

Subsequent Event [Line Items]

Dividend, in usd per share

$ 0.3719

Payable Date

Apr. 01, 2024

Dividends Payable, Date Declared

Mar. 15, 2024

Subsequent Event | 5.625% Preference Shares (AHL PRD)

Subsequent Event [Line Items]

Preferred Stock, Dividend Rate, Percentage

5.625%

Dividend, in usd per share

0.3516

Payable Date

Apr. 01, 2024

Dividends Payable, Date Declared

Mar. 15, 2024

Subsequent Event | 5.625% Preference Shares (AHL PRE)

Subsequent Event [Line Items]

Preferred Stock, Dividend Rate, Percentage

5.625%

Dividend, in usd per share

351.56

Payable Date

Apr. 01, 2024

Dividends Payable, Date Declared

Mar. 15, 2024

Depositary share dividend

$ 0.35156

Schedule I - Investments (Detai

Schedule I - Investments (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Fair Value	$ 6,319.8
Amount at which shown at the balance sheet	6,412.4
Privately-held Investments	475	$ 533
Other investments, at fair value (2)	209.3	221.3
Fair Value, Recurring [Member] | Apollo originating partnership
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Privately-held Investments	112.4	44.8
Other investments, at fair value (2)	15.9	12.7
Fair Value, Recurring [Member] | Apollo real estate fund
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Other investments, at fair value (2)	23.9	$ 25.3
U.S. government
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	1,473.6
Fair Value	1,448.1
Amount at which shown at the balance sheet	1,448.1
U.S. agency
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	7.5
Fair Value	7.2
Amount at which shown at the balance sheet	7.2
Municipal
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	136.9
Fair Value	131.2
Amount at which shown at the balance sheet	131.2
Corporate
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	2,229.9
Fair Value	2,130.8
Amount at which shown at the balance sheet	2,130.8
High yield loans
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	90.8
Fair Value	92.1
Amount at which shown at the balance sheet	92.1
Non-U.S. government-backed corporate
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	115.1
Fair Value	109
Amount at which shown at the balance sheet	109
Non-U.S government
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	315.7
Fair Value	308.6
Amount at which shown at the balance sheet	308.6
Asset-backed
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	936
Fair Value	908.2
Amount at which shown at the balance sheet	908.2
Agency commercial mortgage-backed securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	6.6
Fair Value	5.8
Amount at which shown at the balance sheet	5.8
Agency mortgage-backed
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	544.9
Fair Value	467.3
Amount at which shown at the balance sheet	467.3
Fixed maturities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	5,857
Fair Value	5,608.3
Amount at which shown at the balance sheet	5,608.3
Short-term investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	95.7
Fair Value	95.7
Amount at which shown at the balance sheet	95.7
Catastrophe Bonds
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	1.6
Fair Value	1.6
Amount at which shown at the balance sheet	1.6
Privately-held investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	509.6
Fair Value	437.1
Amount at which shown at the balance sheet	489.9
Equity Method Investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Fair Value	7.6
Amount at which shown at the balance sheet	7.6
Other investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Fair Value	169.6
Amount at which shown at the balance sheet	$ 209.3

Schedule II - Condensed Finan_2

Schedule II - Condensed Financial Information of Registrant Balance Sheets (Details) - USD ($) $ / shares in Units, $ in Thousands				12 Months Ended
	Aug. 13, 2019	Sep. 20, 2016	May 02, 2013	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
ASSETS
Cash and cash equivalents (including cash within consolidated variable interest entities of — $65.7448377 and $75.15054353)				$ 1,028,100	$ 959,200
Right-of-use operating lease assets				61,600	72,800
Other assets				309,600	384,200
Total assets				15,224,800	15,157,300
LIABILITIES
Accrued expenses and other payables (4)				214,400	201,800
Long-term debt				300,000	0
Short-term debt				0	299,900
Operating lease liabilities				86,100	95,500
Total liabilities				12,316,300	12,799,300
SHAREHOLDERS’ EQUITY
60,395,839 ordinary shares of par value $0.01 each (December 31, 2021 — 60,395,839 shares)				604	604
Preference shares, value				753,500	753,500
Additional paid in capital				761,200	761,200
Retained earnings				1,793,500	1,349,000
Unrealized (loss)/gains on investments				(227,600)	(333,200)
Total accumulated other comprehensive (loss)				(400,300)	(506,300)	$ (123,000)
Total shareholders’ equity				2,908,500	2,358,000
Total liabilities and shareholders’ equity				$ 15,224,800	$ 15,157,300
Ordinary shares, issued				60,395,839	60,395,839
Ordinary shares, par value				$ 0.01	$ 0.01
Preference shares, par value				$ 0.0015144558
5.950% Preference Shares (AHL PRC)
SHAREHOLDERS’ EQUITY
Preference shares, issued			11,000,000	11,000,000	11,000,000
Preference shares, rate			5.95%	5.95%	5.95%
Preference shares, par value				$ 0.0015144558	$ 0.0015144558
5.625% Preference Shares (AHL PRD)
SHAREHOLDERS’ EQUITY
Preference shares, issued		10,000,000		10,000,000	10,000,000
Preference shares, rate		5.625%		5.625%	5.625%
Preference shares, par value				$ 0.0015144558	$ 0.0015144558
5.625% Preference Shares (AHL PRE)
SHAREHOLDERS’ EQUITY
Preference shares, issued	10,000,000			10,000,000	10,000,000
Preference shares, rate	5.625%			5.625%	5.625%
Preference shares, par value				$ 0.0015144558	$ 0.0015144558
Parent Company
ASSETS
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0)				$ 43,200	$ 40,500
Cash and cash equivalents (including cash within consolidated variable interest entities of — $65.7448377 and $75.15054353)				43,500	44,400	$ 45,000	$ 92,400
Investments in subsidiaries (1)				3,284,200	2,767,900
Other receivables				4,300	1,700
Right-of-use operating lease assets				1,500	2,000
Other assets				5,600	5,900
Total assets				3,382,300	2,862,400
LIABILITIES
Accrued expenses and other payables (4)				27,800	22,300
Other Liabilities				144,700	180,500
Long-term debt				300,000	0
Short-term debt				0	299,900
Operating lease liabilities				1,300	1,700
Total liabilities				473,800	504,400
SHAREHOLDERS’ EQUITY
60,395,839 ordinary shares of par value $0.01 each (December 31, 2021 — 60,395,839 shares)				600	600
Preference shares, value				753,500	753,500
Additional paid in capital				761,200	761,200
Retained earnings				1,793,500	1,349,000
(Loss)/gain on derivatives				(200)	13,800
Gains on foreign currency translation (start of period)				(186,900)
Gains on foreign currency translation (end of period)				(172,500)	(186,900)
Total accumulated other comprehensive (loss)				(400,300)	(506,300)
Total liabilities and shareholders’ equity				$ 3,382,300	$ 2,862,400

Schedule II - Condensed Finan_3

Schedule II - Condensed Financial Information of Registrant Statements of Operations and Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Condensed Financial Statements, Captions [Line Items]
Dividend income	$ 287.3	$ 198.4	$ 159.2
Other income	0	8.2	14.7
Total revenues	2,966.1	2,890	2,628.9
Expenses
General, administrative and corporate expenses	(503.6)	(494.2)	(418)
Other expenses	0	(20.1)	(10.8)
Income (loss) from operations before income taxes	402.6	(27)	35.1
Income tax benefit/(expense)	132.1	78.1	(5.3)
Net income/(loss)	534.7	51.1	29.8
Other Comprehensive Income:
Change in unrealized (losses)/gains on investments	105.6	(367.8)	(157.9)
Other comprehensive (loss)/income, net of tax	106	(383.3)	(142.7)
Total comprehensive (loss)/income attributable to Aspen Insurance Holdings Limited's ordinary shareholders	640.7	(332.2)	(112.9)
Parent Company
Condensed Financial Statements, Captions [Line Items]
Equity in net earnings/(losses) of subsidiaries and other investments, equity method	304.7	14.2	(90.5)
Dividend income	364.4	121.7	193
Net realized and unrealized investment (losses)/gains	1.1	(4)	(0.5)
Other income	3.2	1	0
Total revenues	673.4	132.9	102
Expenses
General, administrative and corporate expenses	(121.3)	(62.7)	(54.6)
Interest expense	(15.6)	(14.3)	(14.3)
Other expenses	(1.8)	(4.8)	(3.3)
Income (loss) from operations before income taxes	534.7	51.1	29.8
Income tax benefit/(expense)	0	0	0
Net income/(loss)	534.7	51.1	29.8
Other Comprehensive Income:
Change in unrealized (losses)/gains on investments	105.6	(367.8)	(158.6)
Net change from current period hedged transactions	(14)	15.4	(6.2)
Change in foreign currency translation adjustment	14.4	(30.9)	21.4
Other comprehensive (loss)/income, net of tax	106	(383.3)	(143.4)
Total comprehensive (loss)/income attributable to Aspen Insurance Holdings Limited's ordinary shareholders	$ 640.7	$ (332.2)	$ (113.6)

Schedule II - Condensed Finan_4

Schedule II - Condensed Financial Information of Registrant Statements of Cash Flows (Details) - USD ($)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Cash flows (used in)/from operating activities:
Amortization of right-of-use operating lease assets	$ 10,700,000	$ 10,100,000	$ 12,000,000
Interest on operating lease liability	(4,500,000)	(5,400,000)	(5,500,000)
Change in right of lease assets	0	(6,700,000)	400,000
Change in operating lease liabilities	(15,500,000)	(15,500,000)	(17,500,000)
Net cash (used in)/from operating activities	324,700,000	(55,000,000)	524,700,000
Cash flows (used in)/from investing activities:
Proceeds from sales and maturities of fixed income securities — Trading	474,000,000	293,600,000	548,200,000
Net cash (used in)/from investing activities	(172,200,000)	(196,500,000)	(950,300,000)
Cash flows used in/from financing activities:
Repayments of Short-Term Debt	(300,000,000)	0	0
Proceeds from term loan facility	300,000,000	0	0
Dividends paid on ordinary shares	(40,300,000)	(40,000,000)	0
Payments of Ordinary Dividends, Preferred Stock and Preference Stock	49,900,000	44,600,000	44,500,000
Net cash (used in)/from financing activities	(90,200,000)	(84,600,000)	500,000
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect	68,900,000	(354,900,000)	(433,200,000)
Cash and cash equivalents at beginning of period	959,200,000
Cash and cash equivalents at end of period (1)	1,028,100,000	959,200,000
Parent
Cash flows used in/from financing activities:
Proceeds from term loan facility	300,000,000	0	0
Parent Company
Cash flows (used in)/from operating activities:
Net income (excluding equity in net earnings of subsidiaries)	230,000,000	36,900,000	120,300,000
Realized and unrealized losses/(gains)	(14,800,000)	19,700,000	(6,700,000)
(Gain)/loss on derivative contracts	14,000,000	(15,400,000)	6,200,000
Amortization of right-of-use operating lease assets	500,000	500,000	500,000
Interest on operating lease liability	100,000	100,000	100,000
Change in other assets	300,000	2,100,000	(1,600,000)
Change in accrued expenses and other payables	5,500,000	(3,200,000)	1,400,000
Change in intercompany activities	(38,400,000)	41,100,000	35,000,000
Change in right of lease assets	0	0	(2,000,000)
Change in operating lease liabilities	(500,000)	(600,000)	1,500,000
Net cash (used in)/from operating activities	196,700,000	81,200,000	154,700,000
Cash flows (used in)/from investing activities:
(Purchases) of fixed income securities	(8,100,000)	(10,000,000)	(42,600,000)
Proceeds from sales and maturities of fixed income securities — Trading	6,400,000	7,800,000	0
Investment in subsidiaries	(105,700,000)	5,000,000	(115,000,000)
Net cash (used in)/from investing activities	(107,400,000)	2,800,000	(157,600,000)
Cash flows used in/from financing activities:
Repayments of Short-Term Debt	(300,000,000)	0	0
Net cash (used in)/from financing activities	(90,200,000)	(84,600,000)	(44,500,000)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect	(900,000)	(600,000)	(47,400,000)
Cash and cash equivalents at beginning of period	44,400,000	45,000,000	92,400,000
Cash and cash equivalents at end of period (1)	$ 43,500,000	$ 44,400,000	$ 45,000,000

Schedule III - Supplementary _2

Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Deferred Policy Acquisition Costs	$ 296.2	$ 319	$ 290.8
Net Reserves for Losses and LAE	3,232.8	2,813.2	4,313.7	$ 3,970.1
Net Reserves for Unearned Premiums	1,692.8	1,720.2	1,516.2
Net earned premiums	2,614.5	2,688.7	2,410.5
Net Investment Income	275.7	188.1	147.5
Losses and LAE Expenses	1,553	1,680	1,693.3
Policy Acquisition Expenses	380.2	431.8	414.1
Net written premiums	2,581.9	2,896	2,587.7
General and Administrative Expenses	354.5	386.5	333.1
Reinsurance
Deferred Policy Acquisition Costs	201.5	227.2	205.2
Net Reserves for Losses and LAE	1,373.1	1,360.7	2,148.4
Net Reserves for Unearned Premiums	644.5	862.4	688.6
Net earned premiums	1,154.5	1,251.8	1,118.8
Losses and LAE Expenses	611.1	770.3	705.2
Policy Acquisition Expenses	208.6	252.4	221.6
Net written premiums	1,098	1,426.4	1,199
General and Administrative Expenses	120.6	142.5	121.3
Insurance
Deferred Policy Acquisition Costs	94.7	91.8	85.6
Net Reserves for Losses and LAE	1,859.7	1,452.5	2,165.3
Net Reserves for Unearned Premiums	1,048.3	857.8	827.6
Net earned premiums	1,460	1,436.9	1,291.7
Losses and LAE Expenses	941.9	909.7	988.1
Policy Acquisition Expenses	171.6	179.4	192.5
Net written premiums	1,483.9	1,469.6	1,388.7
General and Administrative Expenses	$ 233.9	$ 244	$ 211.8

Schedule IV - Reinsurance Premi

Schedule IV - Reinsurance Premiums Written and Premiums Earned (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Abstract]
Direct	$ 2,446.6	$ 2,531.7	$ 2,341.4
Assumed	1,521	1,807	1,597
Ceded	(1,385.7)	(1,442.7)	(1,350.7)
Net written premiums	2,581.9	2,896	2,587.7
Gross Amount	2,444.8	2,370.8	2,139.1
Assumed From Other Companies	1,562	1,617.2	1,479.2
Ceded	(1,392.3)	(1,299.3)	(1,207.8)
Net premiums earned	$ 2,614.5	$ 2,688.7	$ 2,410.5
Percentage of Amount Assumed to Net	59.70%	60.10%	61.40%
Gross written premiums	$ 3,967.6	$ 4,338.7	$ 3,938.4

Schedule V - Valuation and Qu_2

Schedule V - Valuation and Qualifying Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Premiums receivable from underwriting activities
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning  of Year	$ 25	$ 30.2	$ 34
Charged to Costs and Expenses	(4)	(5.2)	(3.8)
Charged to Other Accounts	0	0	0
Deductions	0	0	0
Balance at End of  Year	21	25	30.2
Reinsurance
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning  of Year	0	0	0
Charged to Costs and Expenses	0
Charged to Other Accounts	0	0	0
Deductions	0	0	0
Balance at End of  Year	$ 0	$ 0	$ 0