Reconciliation of Total Operating Revenue and Operating Income by Segment to Revenue and Net Income (Detail) (USD $) In Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenue	$ 2,568.1	$ 2,540.1	$ 5,116.0	$ 5,076.1
Realized Investment Gains (Losses)	(3.6)	(29.5)	11.6	(3.9)
Operating Income (Loss)	341.6	337.6	656.6	673.2
Revenues	2,564.5	2,510.6	5,127.6	5,072.2
Income Tax Expense (Benefit)	108.2	98.4	213.0	229.8
Net Income (Loss) Attributable to Parent	$ 229.8	$ 209.7	$ 455.2	$ 439.5