Schedule of Unaudited Quarterly Results (Detail) - USD ($) $ / shares in Units, $ 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
Selected Quarterly Financial Information [Abstract]
Premium Income	$ 2,263.3	$ 2,251.8		$ 2,221.0	$ 2,250.0	$ 2,158.4		$ 2,153.6	$ 2,142.2	$ 2,142.9		$ 8,986.1	$ 8,597.1	$ 8,357.7
Net Investment Income	608.6	619.2		623.6	602.3	619.8		609.0	620.5	602.4		2,453.7	2,451.7	2,459.0
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
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
Income (Loss) 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 Income (Loss)	$ 249.1	$ (284.7)	[1]	$ 285.5	$ 273.5	$ 266.9	[2],[4]	$ 252.3	$ 245.1	$ 229.9	[3]	$ 523.4	$ 994.2	$ 931.4
Basic Earnings Per Share	$ 1.15	$ (1.30)	[1]	$ 1.29	$ 1.23	$ 1.19	[2],[4]	$ 1.12	$ 1.08	$ 1.00	[3]	$ 2.38	$ 4.39	$ 3.96
Diluted Earnings Per Share	$ 1.15	$ (1.30)	[1]	$ 1.29	$ 1.23	$ 1.19	[2],[4]	$ 1.12	$ 1.07	$ 1.00	[3]	$ 2.38	$ 4.37	$ 3.95

[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.

[4] Fourth quarter of 2017 net tax benefit of $31.5 million related to the TCJA.