Reserves for Losses and Loss Adjustment Expenses - Reserves for Losses and Loss Adjustment Expenses (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves beginning of the year	$ 3,123.2	$ 2,906.1	$ 2,722.7
Net Ariel Indemnity Limited reserves acquired	0.0	0.0	27.9
Losses and LAE incurred during current calendar year, net of reinsurance:
Current accident year	1,102.2	1,176.3	1,201.1
Prior accident years	64.7	138.3	7.7
Losses and LAE incurred during calendar year, net of reinsurance	1,166.9	1,314.6	1,208.8
Losses and LAE payments made during current calendar year, net of reinsurance:
Current accident year	171.5	180.8	253.4
Prior accident years	828.0	688.4	866.4
Losses and LAE payments made during current calendar year, net of reinsurance:	999.5	869.2	1,119.8
Less:
Divestitures	35.2	0.0	0.0
Reclassified to liabilities held-for-sale	313.0	0.0	0.0
Net reserves ceded to Syndicate 1200 (2)	129.6	0.0	0.0
Loss portfolio transfer:
Reinsurance to close transaction (for years of account 2017 and prior)	0.0	219.7	0.0
Total deductions	1,094.4	219.7	0.0
Change in participation interest	34.3	8.4	32.8
Foreign exchange adjustments	(17.4)	(17.0)	33.7
Net reserves - end of year	2,213.1	3,123.2	2,906.1
Reinsurance recoverables on unpaid losses and LAE, end of year	2,838.5	2,471.8	2,499.9
Reserves for losses and loss adjustment expenses	5,051.6	5,595.0	5,406.0
United States (Federal)
Loss portfolio transfer:
Loss portfolio transfer (for years of account 2019 and 2018)	472.6	0.0	0.0
Syndicate 1200
Loss portfolio transfer:
Loss portfolio transfer (for years of account 2019 and 2018)	$ 144.0	$ 0.0	$ 0.0