Reconciliation of Operating Revenue and Operating Income by Segment to Revenue and Income Before Tax (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Segment Reporting, Revenue Reconciling Item [Line Items]
Net Realized Investment Gain (Loss)	$ 0.8	$ 25.9	$ (14.5)	$ 32.2
Total Revenue	2,703.7	2,656.8	5,351.1	5,271.2
Net Realized Investment Gain (Loss)	0.8	25.9	(14.5)	32.2
Income Before Income Tax	319.8	349.8	622.8	679.6
Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenue	2,702.9	2,630.9	5,365.6	5,239.0
Net Realized Investment Gain (Loss)	0.8	25.9	(14.5)	32.2
Total Revenue	2,703.7	2,656.8	5,351.1	5,271.2
Operating Income	321.9	338.5	643.2	663.4
Net Realized Investment Gain (Loss)	0.8	25.9	(14.5)	32.2
Non-operating Retirement-related Loss	(2.9)	(1.4)	(5.9)	(2.8)
Make-whole Premium on Early Repayment of Debt	0.0	(13.2)	0.0	(13.2)
Income Before Income Tax	$ 319.8	$ 349.8	$ 622.8	$ 679.6