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
	Mar. 31, 2014	Mar. 31, 2013
Segment Reporting [Abstract]
Operating Revenue	$ 2,604.8	$ 2,614.5
Net Realized Investment Gain	6.3	10.3
Total Revenue	2,611.1	2,624.8
Operating Income	321.6	308.4
Net Realized Investment Gain	6.3	10.3
Non-operating Retirement-related Loss	(1.4)	(14.9)
Income Before Income Tax	$ 326.5	$ 303.8