Reconciliation of Operating Revenue and Operating Income by Segment to Revenue and Income Before Tax (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Segment Reporting, Revenue Reconciling Item
Net Realized Investment Gain (Loss)	$ (20.5)	$ (15.3)
Total Revenue	2,725.4	2,647.4
Net Realized Investment Gain (Loss)	(20.5)	(15.3)
Income Before Income Tax	304.2	303.0
Operating Segments
Segment Reporting, Revenue Reconciling Item
Operating Revenue	2,745.9	2,662.7
Net Realized Investment Gain (Loss)	(20.5)	(15.3)
Total Revenue	2,725.4	2,647.4
Operating Income	328.8	321.3
Net Realized Investment Gain (Loss)	(20.5)	(15.3)
Non-operating Retirement-related Loss	(4.1)	(3.0)
Income Before Income Tax	$ 304.2	$ 303.0