Reconciliation of income tax computed at US Federal tax rates (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Income Tax [Abstract]
U.S. Federal Statutory Income Tax Rate, Amount		$ 196.0	$ 472.8	$ 344.4
State and Local Income Taxes, net of Federal Income tax effect, Amount	[1]	1.8	(7.3)	(3.5)
Foreign Tax Effects, Amount		0.1	2.8	(2.0)
Effect of Cross-Border Tax Laws, Amount		3.5	(0.9)	8.4
Tax Credits, Amount		(10.4)	(2.1)	(2.3)
Nondeductible Expense, Amount		(4.8)	(3.9)	(1.5)
Interest on Unrecognized Tax Benefits, Other Adjustments, Amount		11.2	13.2	12.2
Other Adjustments, Amount		(2.4)	(2.4)	0.6
Total Income Tax		$ 195.0	$ 472.2	$ 356.3
Statutory Income Tax, Rate		21.00%	21.00%	21.00%
State and Local Income Taxes, net of Federal Income tax effect, Percent		0.20%	(0.30%)	(0.20%)
Foreign Tax Effects, Percent		0.00%	0.10%	(0.10%)
Effect of Cross-Border Tax Laws, Percent		0.40%	0.00%	0.50%
Tax Credits, Percent		(1.10%)	(0.10%)	(0.10%)
Nondeductible Expense, Percent		(0.50%)	(0.20%)	(0.10%)
Interest on Unrecognized Tax Benefits, percent		1.20%	0.60%	0.70%
Other Adjustments, Percent		(0.30%)	(0.10%)	0.00%
Effective Tax Rate		20.90%	21.00%	21.70%

[1] State taxes in Minnesota, California, and New York made up the majority (greater than 50 percent) of the tax effect in this category for 2025. State taxes in Maine made up the majority (greater than 50 percent) of the tax effect in this category for 2024 and 2023.