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								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenue by Segment									$ 10,282.9	$ 10,168.5	$ 10,079.3
Net Realized Investment Gain (Loss)									(4.9)	24.7	11.7
Revenue	2,604.8	2,545.6	2,564.5	2,563.1	2,593.1	2,527.9	2,510.6	2,561.6	10,278.0	10,193.2	10,091.0
Operating Income by Segment									262.1	1,306.6	1,280.6
Net Realized Investment Gain (Loss)									(4.9)	24.7	11.7
Income Tax Expense (Benefit)									21.8	445.2	439.7
Net Income	$ (425.4)	$ 205.6	$ 229.8	$ 225.4	$ 225.8	$ 220.8	$ 209.7	$ 229.8	$ 235.4	$ 886.1	$ 852.6