Reconciliation of Operating Revenue and Operating Income by Segment to Revenue and Income Before Tax (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Segment Reporting [Abstract]
Operating Revenue	$ 2,627.6	$ 2,588.6	$ 5,232.4	$ 5,203.1
Net Realized Investment Gain	25.9	13.3	32.2	23.6
Total Revenue	2,653.5	2,601.9	5,264.6	5,226.7
Operating Income	335.2	311.1	656.8	619.5
Net Realized Investment Gain	25.9	13.3	32.2	23.6
Non-operating Retirement-related Loss	(1.4)	(12.9)	(2.8)	(27.8)
Costs Related to Early Retirement of Debt	(13.2)	0	(13.2)	0
Income Before Income Tax	$ 346.5	$ 311.5	$ 673.0	$ 615.3