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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting, Revenue Reconciling Item
Total Revenue	$ 2,890.3	$ 2,822.0	$ 5,789.9	$ 5,628.5
Net Realized Investment Gain (Loss)	(2.6)	8.1	(4.8)	19.1
Income Before Income Tax	354.8	361.6	695.9	691.9
Net Realized Investment Gain (Loss)	(2.6)	8.1	(4.8)	19.1
Operating Segments
Segment Reporting, Revenue Reconciling Item
Total Revenue	2,890.3	2,822.0	5,789.9	5,628.5
Net Realized Investment Gain (Loss)	(2.6)	8.1	(4.8)	19.1
Adjusted Operating Revenue	2,892.9	2,813.9	5,794.7	5,609.4
Income Before Income Tax	354.8	361.6	695.9	691.9
Net Realized Investment Gain (Loss)	(2.6)	8.1	(4.8)	19.1
Loss Related to Guaranty Fund Assessment	0.0	0.0	0.0	(20.6)
Adjusted Operating Income	$ 357.4	$ 353.5	$ 700.7	$ 693.4