Reconciliation of Operating Revenue and Operating Income by Segment to Revenue and Income Before Tax (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting, Revenue Reconciling Item
Total Revenue	$ 2,819.1	$ 2,763.3	$ 8,447.6	$ 8,250.0
Net Realized Investment Gain (Loss)	9.8	11.0	28.9	(4.2)
Income Before Income Tax	366.8	332.1	1,058.7	976.5
Net Realized Investment Gain (Loss)	9.8	11.0	28.9	(4.2)
Operating Segments
Segment Reporting, Revenue Reconciling Item
Total Revenue	2,819.1	2,763.3	8,447.6	8,250.0
Net Realized Investment Gain (Loss)	9.8	11.0	28.9	(4.2)
Operating Revenue	2,809.3	2,752.3	8,418.7	8,254.2
Income Before Income Tax	366.8	332.1	1,058.7	976.5
Net Realized Investment Gain (Loss)	9.8	11.0	28.9	(4.2)
Loss Related to Guaranty Fund Assessment	0.0	0.0	(20.6)	0.0
Operating Income	$ 357.0	$ 321.1	$ 1,050.4	$ 980.7