Income Taxes - Reconciliation of Federal Statutory Income Tax Rate - 2025 (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2025	Dec. 31, 2024		Dec. 31, 2023
Amounts
Tax at U.K. federal statutory rate		$ 189.1
Other		1.7
Worldwide changes in prior year unrecognized tax benefits		0.5
Total provision (benefit) for income taxes		$ 107.0	$ 93.3		$ (4.0)
Percentages
Tax at U.K. federal statutory rate		25.00%	25.00%	[1]	23.50%	[1]
Statutory tax rate difference	[2]		(11.50%)		(13.20%)
Excess tax benefits on stock-based compensation			(1.50%)		(0.10%)
Other		0.20%
Worldwide changes in prior year unrecognized tax benefits		0.10%
Effective tax rate		14.10%	13.00%		(0.60%)
Switzerland
Amounts
Statutory tax rate difference		$ (73.1)
Partnership impacts		$ (23.7)
Effective Income Tax Rate Reconciliation, Cantonal Income Taxes, Percent		1.30%
Effective Income Tax Rate Reconciliation, Cantonal Income Taxes, Amount		$ 9.9
Other		$ 3.5
Percentages
Statutory tax rate difference		(9.70%)
Partnership impacts		(3.10%)
Other		0.50%
Other foreign jurisdictions
Amounts
Statutory tax rate difference		$ 2.2
Percentages
Statutory tax rate difference		0.30%
United States
Amounts
Statutory tax rate difference		$ (8.7)
Cantonal income taxes		7.2
Other		6.2
Excess tax benefits on stock-based compensation		$ (7.8)
Percentages
Statutory tax rate difference		(1.20%)
Cantonal income taxes		0.90%
Other		0.80%
Excess tax benefits on stock-based compensation		(1.00%)

[1] The U.K. Finance Act of 2021 increased the statutory tax rate from 19.0% to 25.0%, effective April 1, 2023. Given this change, a prorated U.K. federal statutory income tax rate was utilized for 2023.

[2] The tax effect of international operations consists of non-U.K. jurisdictions.