Reconciliation of Total Revenue and Income Before Income Tax to Operating Revenue and Operating Income (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting, Revenue Reconciling Item
Total Revenue	$ 2,996.3	$ 2,960.0	$ 8,888.6	$ 8,964.3
Net Realized Investment Gain (Loss)	4.4	(26.2)	(105.8)	(32.4)
Income Before Income Tax	299.6	299.4	839.3	1,004.7
Net Realized Investment Gain (Loss)	4.4	(26.2)	(105.8)	(32.4)
Costs Related to Organizational Design Update			(23.3)
Impairment Loss on ROU Asset			(12.7)
Operating Segments
Segment Reporting, Revenue Reconciling Item
Adjusted Operating Revenue	2,991.9	2,986.2	8,994.4	8,996.7
Total Revenue	2,996.3	2,960.0	8,888.6	8,964.3
Net Realized Investment Gain (Loss)	4.4	(26.2)	(105.8)	(32.4)
Income Before Income Tax	299.6	299.4	839.3	1,004.7
Net Realized Investment Gain (Loss)	4.4	(26.2)	(105.8)	(32.4)
Costs Related to Organizational Design Update	(23.3)		(23.3)
Impairment Loss on ROU Asset	0.0	0.0	(12.7)	0.0
Cost Related to Early Retirement of Debt	0.0	(25.2)	0.0	(25.2)
Adjusted Operating Income	$ 318.5	$ 350.8	$ 981.1	$ 1,062.3