Reconciliation of Total Operating Revenue and Operating Income by Segment to Revenue and Net Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenue by Segment	$ 2,606.7	$ 2,569.5	$ 7,825.6	$ 7,685.5
Net Realized Investment Gain (Loss)	21.3	(23.9)	31.6	(12.3)
Revenue	2,628.0	2,545.6	7,857.2	7,673.2
Operating Income by Segment	310.7	320.6	930.0	986.9
Net Realized Investment Gain (Loss)	21.3	(23.9)	31.6	(12.3)
Non-operating Retirement-related Loss	(11.6)	(8.0)	(34.8)	(24.0)
Income Tax	90.2	86.7	266.3	297.4
Net Income	$ 230.2	$ 202.0	$ 660.5	$ 653.2