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, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenue by Segment									$ 10,459.2	$ 10,282.9	$ 10,168.5
Net Realized Investment Gain (Loss)									56.2	(4.9)	24.7
Revenue	2,658.2	2,628.0	2,617.9	2,611.3	2,604.8	2,545.6	2,564.5	2,563.1	10,515.4	10,278.0	10,193.2
Operating Income (Loss)									1,239.7	370.1	1,327.4
Net Realized Investment Gain (Loss)									56.2	(4.9)	24.7
Non-operating Retirement-related Loss									(46.4)	(31.9)	(32.2)
Income Tax Expense (Benefit)									355.1	49.1	441.2
Net Income	$ 233.9	$ 230.2	$ 216.4	$ 213.9	$ (369.0)	$ 202.0	$ 227.6	$ 223.6	$ 894.4	$ 284.2	$ 878.7