Schedule of Income subject to domestic and foreign taxation (Detail) - USD ($) $ in Millions	3 Months Ended													12 Months Ended
	Dec. 31, 2020	[1],[2],[3],[4]	Sep. 30, 2020	[5]	Jun. 30, 2020	[6]	Mar. 31, 2020	Dec. 31, 2019	[7]	Sep. 30, 2019	[8]	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax [Abstract]
Income Before Tax, Domestic														$ 924.7	$ 1,289.5	$ 492.6
Income Before Tax, Foreign														39.3	92.6	135.2
Income Before Income Tax	$ 124.7		$ 299.6		$ 337.6		$ 202.1	$ 377.4		$ 299.4		$ 352.0	$ 353.3	964.0	1,382.1	627.8
Current Tax Expense (Benefit), Domestic														(98.4)	273.6	194.6
Current State and Local Tax Expense (Benefit)														1.5	1.3	(0.6)
Current Tax Expense (Benefit), Foreign														(19.7)	(0.1)	33.4
Income Tax (Benefit) - Current														(116.6)	274.8	227.4
Deferred Tax Expense (Benefit), Federal														250.5	(9.5)	(114.6)
Deferred State and Local Income Tax Expense (Benefit)														1.0	(0.1)	(0.2)
Deferred Tax Expense (Benefit), Foreign														36.1	16.6	(8.2)
Deferred Income Tax Expense (Benefit)														287.6	7.0	(123.0)
Income Tax Expense														$ 171.0	$ 281.8	$ 104.4

[1] Fourth quarter of 2020 amortization of the cost of reinsurance related to the Closed Block individual disability reinsurance transaction of $2.6 million before tax and $2.0 million after tax.

[2] Fourth quarter of 2020 impacts from the Closed Block individual disability reinsurance transaction of $1,305.5 million before tax and $1,031.3 million after tax.

[3] Fourth quarter of 2020 net realized investment gain of $1,302.3 million before tax and $1,028.8 million after tax, related to the transfer of investments for the Closed Block individual disability reinsurance transaction.

[4] Fourth quarter of 2020 reserve increases of $151.5 million and $17.5 million before tax and $119.7 million and $13.8 million after tax related to long-term care and group pension, respectively.

[5] Third quarter of 2020 costs related to organizational design update of $23.3 million before tax and $18.6 million after tax.

[6] Second quarter of 2020 impairment on ROU asset related to an operating lease for office space that we do not plan to continue using to support our general operations of $12.7 million before tax and $10.0 million after tax.

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

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