Parent Company Statement of Income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Condensed Income Statements, Captions
Other Income	$ 261.1	$ 242.9	$ 224.2
Total Revenue	11,991.0	12,013.8	13,162.1
Cost Related to Early Retirement of Debt	4.2	67.3	0.0
Income Tax Expense (Benefit)	317.2	238.8	171.0
Net Income	1,314.2	824.2	793.0
Other Comprehensive Income (Loss), Net of Tax	(3,110.7)	(20.1)	336.9
Comprehensive Income (Loss)	(1,796.5)	804.1	1,129.9
Parent Company
Condensed Income Statements, Captions
Other Income	81.8	53.5	51.7
Total Revenue	1,411.4	1,682.9	1,026.3
Interest and Debt Expense	188.5	185.9	187.1
Cost Related to Early Retirement of Debt	4.2	67.3	0.0
Other Expenses	35.6	39.1	51.1
Operating Expenses, Total	228.3	292.3	238.2
Income Before Income Tax	1,183.1	1,390.6	788.1
Income Tax Expense (Benefit)	14.7	(24.1)	(15.3)
Income of Parent Company	1,168.4	1,414.7	803.4
Equity in Undistributed Earnings (Loss) of Subsidiaries	145.8	(590.5)	(10.4)
Net Income	1,314.2	824.2	793.0
Other Comprehensive Income (Loss), Net of Tax	(3,110.7)	(20.1)	336.9
Comprehensive Income (Loss)	(1,796.5)	804.1	1,129.9
Parent Company | Cash Dividend
Condensed Income Statements, Captions
Dividends from Subsidiaries	1,306.6	909.8	974.6
Parent Company | Non-Cash Dividend
Condensed Income Statements, Captions
Dividends from Subsidiaries	$ 23.0	$ 719.6	$ 0.0