Parent Company Statement of Income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Condensed Income Statements, Captions
Other Income	$ 242.9	$ 224.2	$ 221.2
Total Revenue	12,013.8	13,162.1	11,998.9
Cost Related to Early Retirement of Debt	67.3	0.0	27.3
Income Before Income Tax	1,063.0	964.0	1,382.1
Income Tax Benefit	238.8	171.0	281.8
Net Income	824.2	793.0	1,100.3
Other Comprehensive Income (Loss), Net of Tax	(20.1)	336.9	851.5
Comprehensive Income (Loss)	804.1	1,129.9	1,951.8
Parent Company
Condensed Income Statements, Captions
Other Income	53.5	51.7	63.9
Total Revenue	1,682.9	1,026.3	1,153.3
Interest and Debt Expense	185.9	187.1	173.2
Cost Related to Early Retirement of Debt	67.3	0.0	27.3
Other Expenses	39.1	51.1	53.4
Operating Expenses, Total	292.3	238.2	253.9
Income Before Income Tax	1,390.6	788.1	899.4
Income Tax Benefit	(24.1)	(15.3)	(21.5)
Income of Parent Company	1,414.7	803.4	920.9
Equity in Undistributed Earnings (Loss) of Subsidiaries	(590.5)	(10.4)	179.4
Net Income	824.2	793.0	1,100.3
Other Comprehensive Income (Loss), Net of Tax	(20.1)	336.9	851.5
Comprehensive Income (Loss)	804.1	1,129.9	1,951.8
Parent Company | Cash Dividend
Condensed Income Statements, Captions
Cash Dividends from Subsidiaries	909.8	974.6	1,089.4
Parent Company | Non-Cash Dividend
Condensed Income Statements, Captions
Cash Dividends from Subsidiaries	$ 719.6	$ 0.0	$ 0.0