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		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenue by Segment	$ 2,588.6	$ 2,620.0	$ 5,203.1	$ 5,218.9
Net Realized Investment Gain (Loss)	13.3	(2.1)	23.6	10.3
Revenue	2,601.9	2,617.9	5,226.7	5,229.2
Operating Income (Loss)	311.1	310.1	619.5	619.3
Net Realized Investment Gain (Loss)	13.3	(2.1)	23.6	10.3
Non-operating Retirement-related Loss	(12.9)	(11.6)	(27.8)	(23.2)
Income Tax Expense (Benefit)	(92.9)	(80.0)	(184.1)	(176.1)
Net Income	$ 218.6	$ 216.4	$ 431.2	$ 430.3