Reconciliation of Total Revenue and Income Before Income Tax to Operating Revenue and Operating Income (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Segment Reporting, Revenue Reconciling Item
Total Revenue	$ 2,871.1	$ 2,987.6
Net Realized Investment Gain (Loss)	(144.0)	1.1
Income Before Income Tax	202.1	353.3
Net Realized Investment Gain (Loss)	(144.0)	1.1
Operating Segments
Segment Reporting, Revenue Reconciling Item
Adjusted Operating Revenue	3,015.1	2,986.5
Total Revenue	2,871.1	2,987.6
Net Realized Investment Gain (Loss)	(144.0)	1.1
Income Before Income Tax	202.1	353.3
Net Realized Investment Gain (Loss)	(144.0)	1.1
Adjusted Operating Income	$ 346.1	$ 352.2