Reconciliation of Total Operating Revenue and Operating Income by Segment to Revenue and Net Income (Detail) (USD $) In Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenue	$ 2,569.5	$ 2,526.8	$ 7,685.5	$ 7,602.9
Realized Investment Gains (Losses)	(23.9)	1.1	(12.3)	(2.8)
Revenues	2,545.6	2,527.9	7,673.2	7,600.1
Operating Income (Loss)	317.2	325.4	973.8	998.6
Income Tax Expense (Benefit)	87.7	105.7	300.7	335.5
Net Income (Loss) Attributable to Parent	$ 205.6	$ 220.8	$ 660.8	$ 660.3