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		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Segment Reporting, Revenue Reconciling Item [Line Items]
Net Realized Investment Gain (Loss)	$ 1.2	$ (26.1)	$ 33.4	$ (2.5)
Total Revenue	2,609.4	2,540.9	7,874.0	7,767.6
Net Realized Investment Gain (Loss)	1.2	(26.1)	33.4	(2.5)
Income Before Income Tax	312.1	284.1	985.1	899.4
Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenue	2,608.2	2,567.0	7,840.6	7,770.1
Net Realized Investment Gain (Loss)	1.2	(26.1)	33.4	(2.5)
Total Revenue	2,609.4	2,540.9	7,874.0	7,767.6
Operating Income	312.3	312.9	969.1	932.4
Net Realized Investment Gain (Loss)	1.2	(26.1)	33.4	(2.5)
Non-operating Retirement-related Loss	(1.4)	(2.7)	(4.2)	(30.5)
Costs Related to Early Retirement of Debt	0	0	(13.2)	0
Income Before Income Tax	$ 312.1	$ 284.1	$ 985.1	$ 899.4