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, 2017		Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting, Revenue Reconciling Item
Total Revenue	$ 2,839.2		$ 2,819.1	$ 2,822.0	$ 2,806.5		$ 2,796.5	$ 2,763.3	$ 2,761.3	$ 2,725.4	$ 11,286.8	$ 11,046.5	$ 10,731.3
Net Realized Investment Gain (Loss)	11.4		9.8	8.1	11.0		28.4	11.0	5.3	(20.5)	40.3	24.2	(43.8)
Income Before Income Tax	345.3	[1]	366.8	361.6	330.3	[2]	371.2	332.1	340.2	304.2	1,404.0	1,347.7	1,238.3
Net Realized Investment Gain (Loss)	$ 11.4		$ 9.8	$ 8.1	$ 11.0		$ 28.4	$ 11.0	$ 5.3	$ (20.5)	40.3	24.2	(43.8)
Unclaimed Death Benefit Reserve Increase Before Taxes											(39.0)
Operating Segments
Segment Reporting, Revenue Reconciling Item
Total Revenue											11,286.8	11,046.5	10,731.3
Net Realized Investment Gain (Loss)											40.3	24.2	(43.8)
Adjusted Operating Revenue											11,246.5	11,022.3	10,775.1
Income Before Income Tax											1,404.0	1,347.7	1,238.3
Net Realized Investment Gain (Loss)											40.3	24.2	(43.8)
Loss from Guaranty Fund Assessment											(20.6)	0.0	0.0
Unclaimed Death Benefit Reserve Increase Before Taxes											(39.0)	0.0	0.0
Adjusted Operating Income											$ 1,423.3	$ 1,323.5	$ 1,282.1

[1] Fourth quarter of 2017 reserve increase of $39.0 million before tax and $25.4 million after tax related to unclaimed death benefits.

[2] First quarter of 2017 loss from a guaranty fund assessment related to an unaffiliated insurer that was declared insolvent of $20.6 million before tax and $13.4 million after tax.