Reconciliation of Total Revenue and Income Before Income Tax to Operating Revenue and Operating Income (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting, Revenue Reconciling Item
Total Revenue	$ 3,021.2	$ 3,016.7	$ 5,892.3	$ 6,004.3
Net Realized Investment Gain (Loss)	33.8	(7.3)	(110.2)	(6.2)
Income Before Income Tax	337.6	352.0	539.7	705.3
Net Realized Investment Gain (Loss)	33.8	(7.3)	(110.2)	(6.2)
Impairment Loss on ROU Asset			12.7
Operating Segments
Segment Reporting, Revenue Reconciling Item
Adjusted Operating Revenue	2,987.4	3,024.0	6,002.5	6,010.5
Total Revenue	3,021.2	3,016.7	5,892.3	6,004.3
Net Realized Investment Gain (Loss)	33.8	(7.3)	(110.2)	(6.2)
Income Before Income Tax	337.6	352.0	539.7	705.3
Net Realized Investment Gain (Loss)	33.8	(7.3)	(110.2)	(6.2)
Impairment Loss on ROU Asset	(12.7)	0.0	(12.7)	0.0
Adjusted Operating Income	$ 316.5	$ 359.3	$ 662.6	$ 711.5