Schedule of Unaudited Quarterly Results (Detail) - USD ($) $ / shares in Units, $ 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
Selected Quarterly Financial Information [Abstract]
Premium Income	$ 2,352.6		$ 2,331.2		$ 2,343.1	$ 2,338.7	$ 2,263.3	$ 2,251.8		$ 2,221.0	$ 2,250.0	$ 9,365.6	$ 8,986.1	$ 8,597.1
Net Investment Income	616.3		599.4		624.9	594.7	608.6	619.2		623.6	602.3	2,435.3	2,453.7	2,451.7
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
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
Income (Loss) 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 Income (Loss)	$ 296.2	[1]	$ 242.0	[2]	$ 281.2	$ 280.9	$ 249.1	$ (284.7)	[3]	$ 285.5	$ 273.5	$ 1,100.3	$ 523.4	$ 994.2
Basic Earnings Per Share	$ 1.44	[1]	$ 1.16	[2]	$ 1.33	$ 1.31	$ 1.15	$ (1.30)	[3]	$ 1.29	$ 1.23	$ 5.25	$ 2.38	$ 4.39
Diluted Earnings Per Share	$ 1.44	[1]	$ 1.16	[2]	$ 1.33	$ 1.31	$ 1.15	$ (1.30)	[3]	$ 1.29	$ 1.23	$ 5.24	$ 2.38	$ 4.37

[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.