Parent Company Statement of Income (Detail) - USD ($) $ in Millions	3 Months Ended											12 Months Ended
	Dec. 31, 2019		Sep. 30, 2019		Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018		Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Condensed Income Statements, Captions
Other Income												$ 221.2	$ 198.2	$ 197.7
Total Revenue	$ 3,034.6		$ 2,960.0		$ 3,016.7	$ 2,987.6	$ 2,880.8	$ 2,927.8		$ 2,890.3	$ 2,899.6	11,998.9	11,598.5	11,286.8
Cost Related to Early Retirement of Debt												27.3	0.0	0.0
Income Before Income Tax	377.4	[1]	299.4	[2]	352.0	353.3	309.7	(377.8)	[3]	354.8	341.1	1,382.1	627.8	1,404.0
Income Tax Expense (Benefit)												281.8	104.4	409.8
Net Income	$ 296.2	[1]	$ 242.0	[2]	$ 281.2	$ 280.9	$ 249.1	$ (284.7)	[3]	$ 285.5	$ 273.5	1,100.3	523.4	994.2
Other Comprehensive Income (Loss), Net of Tax												851.5	(924.2)	178.5
Comprehensive Income (Loss)												1,951.8	(400.8)	1,172.7
Parent Company
Condensed Income Statements, Captions
Cash Dividends from Subsidiaries												1,089.4	1,135.4	830.2
Other Income												63.9	66.6	66.6
Total Revenue												1,153.3	1,202.0	896.8
Interest and Debt Expense												173.2	161.4	154.2
Cost Related to Early Retirement of Debt												27.3	0.0	0.0
Other Expenses												53.4	53.4	37.1
Operating Expenses, Total												253.9	214.8	191.3
Income Before Income Tax												899.4	987.2	705.5
Income Tax Expense (Benefit)												(21.5)	(1.7)	115.3
Income of Parent Company												920.9	988.9	590.2
Equity in Undistributed Earnings (Loss) of Subsidiaries												179.4	(465.5)	404.0
Net Income												1,100.3	523.4	994.2
Other Comprehensive Income (Loss), Net of Tax												851.5	(924.2)	178.5
Comprehensive Income (Loss)												$ 1,951.8	$ (400.8)	$ 1,172.7

[1] Fourth quarter of 2019 cost related to the early retirement of debt of $2.1 million before tax and $1.7 million after tax

[2] Third quarter of 2019 cost related to the early retirement of debt of $25.2 million before tax and $19.9 million after tax.

[3] Third quarter of 2018 reserve increase of $750.8 million before tax and $593.1 million after tax related to long-term care.