Parent Company Statement of Income (Detail) - USD ($) $ in Millions	3 Months Ended										12 Months Ended
	Dec. 31, 2017		Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Income Statements, Captions
Other Income											$ 197.7	$ 205.6	$ 211.5
Total Revenue	$ 2,839.2		$ 2,819.1	$ 2,822.0	$ 2,806.5		$ 2,796.5	$ 2,763.3	$ 2,761.3	$ 2,725.4	11,286.8	11,046.5	10,731.3
Income Before Income Tax	345.3	[1]	366.8	361.6	330.3	[2]	371.2	332.1	340.2	304.2	1,404.0	1,347.7	1,238.3
Income Tax Expense (Benefit)											409.8	416.3	371.2
Net Income	$ 266.9	[1],[3]	$ 252.3	$ 245.1	$ 229.9	[2]	$ 248.0	$ 236.0	$ 236.8	$ 210.6	994.2	931.4	867.1
Net Other Comprehensive Income (Loss)											178.5	(67.1)	(150.3)
Comprehensive Income											1,172.7	864.3	716.8
Parent Company
Condensed Income Statements, Captions
Cash Dividends from Subsidiaries											830.2	676.5	671.8
Other Income											66.6	58.4	52.8
Total Revenue											896.8	734.9	724.6
Interest and Debt Expense											154.2	160.1	137.5
Other Expenses											37.1	32.3	16.7
Total Expenses											191.3	192.4	154.2
Income Before Income Tax											705.5	542.5	570.4
Income Tax Expense (Benefit)											115.3	(20.2)	(16.2)
Income of Parent Company											590.2	562.7	586.6
Equity in Undistributed Earnings of Subsidiaries											404.0	368.7	280.5
Net Income											994.2	931.4	867.1
Net Other Comprehensive Income (Loss)											178.5	(67.1)	(150.3)
Comprehensive Income											$ 1,172.7	$ 864.3	$ 716.8

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

[2] 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.

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