Parent Company Statement of Income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Condensed Income Statements, Captions
Other Income	$ 294.5	$ 279.2	$ 261.1
Total Revenue	12,887.3	12,385.9	11,984.1
Cost Related to Early Retirement of Debt	0.0	0.0	4.2
Income Tax Expense (Benefit)	472.2	356.3	342.8
Net Income	1,779.1	1,283.8	1,407.2
Other Comprehensive Income, Net of Tax	784.3	140.3	1,716.3
Comprehensive Income	2,563.4	1,424.1	3,123.5
Parent Company
Condensed Income Statements, Captions
Other Income	100.5	90.3	81.8
Total Revenue	1,512.2	1,671.4	1,411.4
Interest and Debt Expense	201.1	194.8	188.5
Cost Related to Early Retirement of Debt	0.0	0.0	4.2
Other Expenses	70.8	54.2	35.6
Operating Expenses, Total	271.9	249.0	228.3
Income of Parent Company Before Income Tax	1,240.3	1,422.4	1,183.1
Income Tax Expense (Benefit)	(33.7)	(9.5)	14.7
Income of Parent Company	1,274.0	1,431.9	1,168.4
Equity in Undistributed Earnings (Loss) of Subsidiaries	505.1	(148.1)	238.8
Net Income	1,779.1	1,283.8	1,407.2
Other Comprehensive Income, Net of Tax	784.3	140.3	1,716.3
Comprehensive Income	2,563.4	1,424.1	3,123.5
Parent Company | Cash Dividend
Condensed Income Statements, Captions
Dividends from Subsidiaries	1,378.7	1,581.1	1,306.6
Parent Company | Non-Cash Dividend
Condensed Income Statements, Captions
Dividends from Subsidiaries	$ 33.0	$ 0.0	$ 23.0