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		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Reconciliation of Total Revenue to Adjusted Operating Revenue
Total Revenue	$ 3,016.7	$ 2,890.3	$ 6,004.3	$ 5,789.9
Net Realized Investment Gain (Loss)	(7.3)	(2.6)	(6.2)	(4.8)
Reconciliation of Income Before Income Tax to Adjusted Operating Income
Income Before Income Tax	352.0	354.8	705.3	695.9
Net Realized Investment Gain (Loss)	(7.3)	(2.6)	(6.2)	(4.8)
Operating Segments
Reconciliation of Total Revenue to Adjusted Operating Revenue
Total Revenue	3,016.7	2,890.3	6,004.3	5,789.9
Net Realized Investment Gain (Loss)	(7.3)	(2.6)	(6.2)	(4.8)
Adjusted Operating Revenue	3,024.0	2,892.9	6,010.5	5,794.7
Reconciliation of Income Before Income Tax to Adjusted Operating Income
Income Before Income Tax	352.0	354.8	705.3	695.9
Net Realized Investment Gain (Loss)	(7.3)	(2.6)	(6.2)	(4.8)
Adjusted Operating Income	$ 359.3	$ 357.4	$ 711.5	$ 700.7