Business and Significant Accounting Policies - Additional Information (Detail) $ in Millions			11 Months Ended	12 Months Ended
	Nov. 25, 2020 USD ($)	Apr. 30, 2020 USD ($)	Nov. 30, 2021 property	Dec. 31, 2021 USD ($) segment foundation trust	Dec. 31, 2020 USD ($) property	Dec. 31, 2019 USD ($) property	Oct. 01, 2021 USD ($)	Dec. 31, 2018 USD ($)
Business And Significant Accounting Policies [Line Items]
Number of reportable segments | segment				2
Percentage of contractual participation				100.00%
Number of statutory trusts | trust				11
Number of charitable foundations | foundation				1
Estimated liability for return of premiums				$ 4.7	$ 1.5
Estimated amount of premiums receivables due				0.1	0.1
Impairment of goodwill and intangibles					0.0	$ 15.6
Intangible assets derecognized				43.2	0.0	15.6
Amortization expense				0.0	1.1	1.7
Accumulated depreciation for property and equipment				158.8	163.3
Net book value of property and equipment				67.5	130.4
Depreciation expense				23.9	$ 24.1	$ 24.4
Number of properties held for sale | property						4
Number of properties sold | property			2		2
Assets held for sale				0.0	$ 7.7
Foreign currency translation adjustment gain (loss)				(35.3)	(37.9)
Shareholders' equity				(1,735.2)	(1,857.8)	$ (1,763.7)		$ (1,735.0)
ArgoGlobal Syndicate 1200
Business And Significant Accounting Policies [Line Items]
Carrying amount in excess of fair value				43.2
Impairment of goodwill and intangibles				28.7
Indefinite-lived intangible asset carrying value				17.3			$ 60.5
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Trident
Business And Significant Accounting Policies [Line Items]
Goodwill derecognized		$ 4.9
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Ariel Re
Business And Significant Accounting Policies [Line Items]
Goodwill derecognized	$ 9.2
Intangible assets derecognized	$ 30.2
Disposal Group, Held-for-sale, Not Discontinued Operations
Business And Significant Accounting Policies [Line Items]
Assets held for sale					7.7
Disposal Group, Held-for-sale, Not Discontinued Operations | Other Corporate Expense
Business And Significant Accounting Policies [Line Items]
Reclassification to "Assets held for sale" pretax gain (loss)				0.3	(0.8)	(3.7)
Retained Earnings
Business And Significant Accounting Policies [Line Items]
Shareholders' equity				(636.4)	(684.1)	(793.7)		(850.9)
AOCI Attributable to Parent
Business And Significant Accounting Policies [Line Items]
Shareholders' equity				$ 22.7	$ (58.6)	(2.8)		$ 78.1
Cumulative Effect, Period of Adoption, Adjustment
Business And Significant Accounting Policies [Line Items]
Shareholders' equity						2.2
Cumulative Effect, Period of Adoption, Adjustment | Retained Earnings
Business And Significant Accounting Policies [Line Items]
Shareholders' equity						7.9
Cumulative Effect, Period of Adoption, Adjustment | AOCI Attributable to Parent
Business And Significant Accounting Policies [Line Items]
Shareholders' equity						$ (5.7)
Minimum
Business And Significant Accounting Policies [Line Items]
Useful life				3 years
Maximum
Business And Significant Accounting Policies [Line Items]
Useful life				39 years