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, 2019		Sep. 30, 2019		Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018		Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting, Revenue Reconciling Item
Total Revenue	$ 3,034.6		$ 2,960.0		$ 3,016.7	$ 2,987.6	$ 2,880.8	$ 2,927.8		$ 2,890.3	$ 2,899.6	$ 11,998.9	$ 11,598.5	$ 11,286.8
Net Realized Investment Gain (Loss)	9.2		(26.2)		(7.3)	1.1	(41.4)	6.7		(2.6)	(2.2)	(23.2)	(39.5)	40.3
Income Before Income Tax	377.4	[1]	299.4	[2]	352.0	353.3	309.7	(377.8)	[3]	354.8	341.1	1,382.1	627.8	1,404.0
Net Realized Investment Gain (Loss)	$ 9.2		$ (26.2)		$ (7.3)	$ 1.1	$ (41.4)	$ 6.7		$ (2.6)	$ (2.2)	(23.2)	(39.5)	40.3
Unclaimed Death Benefit Reserve Increase														(39.0)
Operating Segments
Segment Reporting, Revenue Reconciling Item
Adjusted Operating Revenue												12,022.1	11,638.0	11,246.5
Total Revenue												11,998.9	11,598.5	11,286.8
Net Realized Investment Gain (Loss)												(23.2)	(39.5)	40.3
Income Before Income Tax												1,382.1	627.8	1,404.0
Net Realized Investment Gain (Loss)												(23.2)	(39.5)	40.3
Long-term Care Reserve Increase												0.0	(750.8)	0.0
Gain (Loss) on Extinguishment of Debt												27.3	0.0	0.0
Loss from Guaranty Fund Assessment												0.0	0.0	(20.6)
Unclaimed Death Benefit Reserve Increase												0.0	0.0	(39.0)
Adjusted Operating Income												$ 1,432.6	$ 1,418.1	$ 1,423.3

[1] Fourth quarter of 2019 cost related to the early retirement of debt of $2.1 million before tax and $1.7 million after tax

[2] Third quarter of 2019 cost related to the early retirement of debt of $25.2 million before tax and $19.9 million after tax.

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