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, 2018	Sep. 30, 2018		Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017		Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting, Revenue Reconciling Item
Total Revenue	$ 2,880.8	$ 2,927.8		$ 2,890.3	$ 2,899.6	$ 2,839.2		$ 2,819.1	$ 2,822.0	$ 2,806.5		$ 11,598.5	$ 11,286.8	$ 11,046.5
Net Realized Investment Gain (Loss)	(41.4)	6.7		(2.6)	(2.2)	11.4		9.8	8.1	11.0		(39.5)	40.3	24.2
Income Before Income Tax	309.7	(377.8)	[1]	354.8	341.1	345.3	[2]	366.8	361.6	330.3	[3]	627.8	1,404.0	1,347.7
Net Realized Investment Gain (Loss)	$ (41.4)	$ 6.7		$ (2.6)	$ (2.2)	$ 11.4		$ 9.8	$ 8.1	$ 11.0		(39.5)	40.3	24.2
Unclaimed Death Benefit Reserve Increase Before Taxes													(39.0)
Operating Segments
Segment Reporting, Revenue Reconciling Item
Adjusted Operating Revenue												11,638.0	11,246.5	11,022.3
Total Revenue												11,598.5	11,286.8	11,046.5
Net Realized Investment Gain (Loss)												(39.5)	40.3	24.2
Income Before Income Tax												627.8	1,404.0	1,347.7
Net Realized Investment Gain (Loss)												(39.5)	40.3	24.2
Loss from Guaranty Fund Assessment												0.0	(20.6)	0.0
Unclaimed Death Benefit Reserve Increase Before Taxes												0.0	(39.0)	0.0
Reserve Increase for Long-term Care, Before Tax												(750.8)	0.0	0.0
Adjusted Operating Income												$ 1,418.1	$ 1,423.3	$ 1,323.5

[1] Third quarter of 2018 reserve increase of $750.8 million before tax and $593.1 million after tax related to long-term care.

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

[3] 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.