Parent Company Statement of Income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Condensed Income Statements, Captions
Other Income	$ 318.4	$ 294.5	$ 279.2
Total Revenue	13,075.5	12,887.3	12,385.9
Income Tax Expense (Benefit)	195.0	472.2	356.3
Net Income	738.5	1,779.1	1,283.8
Other Comprehensive Income, Net of Tax	715.2	784.3	140.3
Comprehensive Income	1,453.7	2,563.4	1,424.1
Parent Company
Condensed Income Statements, Captions
Other Income	145.6	100.5	90.3
Total Revenue	1,939.8	1,512.2	1,671.4
Interest and Debt Expense	208.8	201.1	194.8
Other Expenses	158.9	70.8	54.2
Operating Expenses, Total	367.7	271.9	249.0
Income of Parent Company Before Income Tax	1,572.1	1,240.3	1,422.4
Income Tax Expense (Benefit)	(42.1)	(33.7)	(9.5)
Income of Parent Company	1,614.2	1,274.0	1,431.9
Equity in Undistributed Earnings (Loss) of Subsidiaries	(875.7)	505.1	(148.1)
Net Income	738.5	1,779.1	1,283.8
Other Comprehensive Income, Net of Tax	715.2	784.3	140.3
Comprehensive Income	1,453.7	2,563.4	1,424.1
Parent Company | Cash Dividend
Condensed Income Statements, Captions
Dividends from Subsidiaries	1,643.1	1,378.7	1,581.1
Parent Company | Non-Cash Dividend
Condensed Income Statements, Captions
Dividends from Subsidiaries	$ 151.1	$ 33.0	$ 0.0