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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Segment Reporting, Revenue Reconciling Item
Total Revenue	$ 2,763.3	$ 2,657.8	$ 8,250.0	$ 8,008.9
Net Realized Investment Gain (Loss)	11.0	(26.6)	(4.2)	(41.1)
Income Before Income Tax	332.1	298.9	976.5	921.7
Net Realized Investment Gain (Loss)	11.0	(26.6)	(4.2)	(41.1)
Operating Segments
Segment Reporting, Revenue Reconciling Item
Total Revenue	2,763.3	2,657.8	8,250.0	8,008.9
Net Realized Investment Gain (Loss)	11.0	(26.6)	(4.2)	(41.1)
Operating Revenue	2,752.3	2,684.4	8,254.2	8,050.0
Income Before Income Tax	332.1	298.9	976.5	921.7
Net Realized Investment Gain (Loss)	11.0	(26.6)	(4.2)	(41.1)
Non-operating Retirement-related Loss	(4.1)	(3.0)	(12.2)	(8.9)
Operating Income	$ 325.2	$ 328.5	$ 992.9	$ 971.7