Reconciliation of Total Revenue and Income Before Income Tax to Operating Revenue and Operating Income (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting, Revenue Reconciling Item
Total Revenue	$ 2,796.5	$ 2,763.3	$ 2,761.3	$ 2,725.4	$ 2,722.4	$ 2,657.8	$ 2,703.7	$ 2,647.4	$ 11,046.5	$ 10,731.3	$ 10,524.5
Net Realized Investment Gain (Loss)	28.4	11.0	5.3	(20.5)	(2.7)	(26.6)	0.8	(15.3)	24.2	(43.8)	16.1
Income Before Income Tax	371.2	332.1	340.2	304.2	316.6	298.9	319.8	303.0	1,347.7	1,238.3	542.0
Net Realized Investment Gain (Loss)	$ 28.4	$ 11.0	$ 5.3	$ (20.5)	$ (2.7)	$ (26.6)	$ 0.8	$ (15.3)	24.2	(43.8)	16.1
Operating Segments
Segment Reporting, Revenue Reconciling Item
Total Revenue									11,046.5	10,731.3	10,524.5
Net Realized Investment Gain (Loss)									24.2	(43.8)	16.1
Operating Revenue									11,022.3	10,775.1	10,508.4
Income Before Income Tax									1,347.7	1,238.3	542.0
Net Realized Investment Gain (Loss)									24.2	(43.8)	16.1
Non-operating Retirement-related Loss									(16.3)	(11.9)	(70.0)
Make-whole Premium on Early Repayment of Debt									0.0	0.0	(13.2)
Reserve Increase for Long-term Care, Before Tax									0.0	0.0	(698.2)
Operating Income									$ 1,339.8	$ 1,294.0	$ 1,307.3