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, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Segment Reporting, Revenue Reconciling Item
Net Realized Investment Gain (Loss)	$ (26.6)	$ 1.2	$ (41.1)	$ 33.4
Total Revenue	2,657.8	2,614.3	8,008.9	7,885.5
Net Realized Investment Gain (Loss)	(26.6)	1.2	(41.1)	33.4
Income Before Income Tax	298.9	317.0	921.7	996.6
Operating Segments
Segment Reporting, Revenue Reconciling Item
Operating Revenue	2,684.4	2,613.1	8,050.0	7,852.1
Net Realized Investment Gain (Loss)	(26.6)	1.2	(41.1)	33.4
Total Revenue	2,657.8	2,614.3	8,008.9	7,885.5
Operating Income	328.5	317.2	971.7	980.6
Net Realized Investment Gain (Loss)	(26.6)	1.2	(41.1)	33.4
Non-operating Retirement-related Loss	(3.0)	(1.4)	(8.9)	(4.2)
Make-whole Premium on Early Repayment of Debt	0.0	0.0	0.0	(13.2)
Income Before Income Tax	$ 298.9	$ 317.0	$ 921.7	$ 996.6