Reconciliation of Total Revenue and Income before Income Tax by Segment to Adjusted Operating Revenue and Adjusted Operating Income (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Reconciliation of Total Revenue to Adjusted Operating Revenue
Total Revenue	$ 2,960.0	$ 2,927.8	$ 8,964.3	$ 8,717.7
Net Realized Investment Gain (Loss)	(26.2)	6.7	(32.4)	1.9
Reconciliation of Income (Loss) Before Income Tax to Adjusted Operating Income
Income (Loss) Before Income Tax Expense (Benefit)	299.4	(377.8)	1,004.7	318.1
Net Realized Investment Gain (Loss)	(26.2)	6.7	(32.4)	1.9
Cost Related to Early Retirement of Debt	(25.2)	0.0	(25.2)	0.0
Long-term Care Reserve Increase, Before Tax				750.8
Operating Segments
Reconciliation of Total Revenue to Adjusted Operating Revenue
Total Revenue	2,960.0	2,927.8	8,964.3	8,717.7
Net Realized Investment Gain (Loss)	(26.2)	6.7	(32.4)	1.9
Adjusted Operating Revenue	2,986.2	2,921.1	8,996.7	8,715.8
Reconciliation of Income (Loss) Before Income Tax to Adjusted Operating Income
Income (Loss) Before Income Tax Expense (Benefit)	299.4	(377.8)	1,004.7	318.1
Net Realized Investment Gain (Loss)	(26.2)	6.7	(32.4)	1.9
Cost Related to Early Retirement of Debt	(25.2)	0.0	(25.2)	0.0
Long-term Care Reserve Increase, Before Tax	0.0	750.8	0.0	750.8
Adjusted Operating Income	$ 350.8	$ 366.3	$ 1,062.3	$ 1,067.0