Business and Significant Accounting Policies (Details) $ / shares in Units, $ in Millions				3 Months Ended			9 Months Ended		20 Months Ended
	Sep. 25, 2024 USD ($) $ / shares shares	Jul. 01, 2024 USD ($)	Feb. 02, 2023 USD ($)	Sep. 30, 2024 USD ($) $ / shares shares		Mar. 31, 2023 USD ($)	Sep. 30, 2024 USD ($) $ / shares shares		Sep. 30, 2024 USD ($) $ / shares shares	Sep. 24, 2024 $ / shares shares	Dec. 31, 2023 $ / shares shares
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Common shares, shares authorized (in shares) | shares	1,000			1,000			1,000		1,000	2,000,000,000	2,000,000,000
Common shares, par value (in dollars per share) | $ / shares	$ 0.01			$ 0.01			$ 0.01		$ 0.01	$ 1.00	$ 1.00
Preferred shares, shares authorized (in shares) | shares	10,000			10,000			10,000		10,000	30,000,000	30,000,000
Purchase accounting adjustments				$ 15.3	[1]		$ 15.3	[2]
AGIH Merger Sub, Inc.
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Business Acquisition, Stock Conversion Ratio	0.000001
Initial bargain purchase gain	$ 48.9
Purchase accounting adjustments				15.3
Bargain purchase				$ 33.6
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Argo Global Holdings Ltd
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Final consideration paid		$ 86.7
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Argo Underwriting Agency Limited
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash consideration received from sale			$ 161.3
Proceeds from sale of Trident assets			130.7
Consideration placed in escrow			$ 30.6
Consideration placed in escrow, term			2 years
Consideration placed in escrow that has been released to the company									$ 13.1
Loss on disposal						$ 20.3

[1] (1) Impact of measurement period adjustment on bargain purchase in connection with the Merger. Refer to Note 1, “Business and Significant Accounting Policies” for additional information.

[2] (2) Refer to the Merger with AGIH Merger Sub, Inc. in Note 1, “Business and Significant Accounting Policies” for additional information.