Parent Company Statement of Income (Detail) - USD ($) $ in Millions	3 Months Ended											12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018		Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017		Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Income Statements, Captions
Other Income												$ 198.2	$ 197.7	$ 205.6
Total Revenue	$ 2,880.8	$ 2,927.8		$ 2,890.3	$ 2,899.6	$ 2,839.2		$ 2,819.1	$ 2,822.0	$ 2,806.5		11,598.5	11,286.8	11,046.5
Income Before Income Tax	309.7	(377.8)	[1]	354.8	341.1	345.3	[2]	366.8	361.6	330.3	[3]	627.8	1,404.0	1,347.7
Income Tax Expense (Benefit)												104.4	409.8	416.3
Net Income	$ 249.1	$ (284.7)	[1]	$ 285.5	$ 273.5	$ 266.9	[2],[4]	$ 252.3	$ 245.1	$ 229.9	[3]	523.4	994.2	931.4
Other Comprehensive Income (Loss), Net of Tax												(924.2)	178.5	(67.1)
Comprehensive Income (Loss)												(400.8)	1,172.7	864.3
Parent Company
Condensed Income Statements, Captions
Cash Dividends from Subsidiaries												1,135.4	830.2	676.5
Other Income												66.6	66.6	58.4
Total Revenue												1,202.0	896.8	734.9
Interest and Debt Expense												161.4	154.2	160.1
Other Expenses												53.4	37.1	32.3
Total Expenses												214.8	191.3	192.4
Income Before Income Tax												987.2	705.5	542.5
Income Tax Expense (Benefit)												(1.7)	115.3	(20.2)
Income of Parent Company												988.9	590.2	562.7
Equity in Undistributed Earnings (Loss) of Subsidiaries												(465.5)	404.0	368.7
Net Income												523.4	994.2	931.4
Other Comprehensive Income (Loss), Net of Tax												(924.2)	178.5	(67.1)
Comprehensive Income (Loss)												$ (400.8)	$ 1,172.7	$ 864.3

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

[2] Fourth quarter of 2017 reserve increase of $39.0 million before tax and $25.4 million after tax related to unclaimed death benefits.

[3] First quarter of 2017 loss from a guaranty fund assessment related to an unaffiliated insurer that was declared insolvent of $20.6 million before tax and $13.4 million after tax.

[4] Fourth quarter of 2017 net tax benefit of $31.5 million related to the TCJA.