Segment Information Deferred Acquisition Costs by Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	$ 2,324.0
Deferred Policy Acquisition Costs, Amortization Expense	(606.1)	$ (609.9)	$ (565.5)
Deferred Acquisition Costs at End of Year	2,272.6	2,324.0
Operating Segments
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	2,324.0	2,309.4	2,184.6
Deferral of Acquisition Costs	576.2	658.6	668.0
Deferred Policy Acquisition Costs, Amortization Expense	(606.1)	(609.9)	(565.5)
Adjustment Related to Unrealized Investment Gains and Losses	(22.4)	(34.8)	23.5
Deferred Policy Acquisition Costs Foreign Currency Translation Adjustment And Other	0.9	0.7	(1.2)
Deferred Acquisition Costs at End of Year	2,272.6	2,324.0	2,309.4
Operating Segments | Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	1,223.0	1,239.4	1,205.4
Deferral of Acquisition Costs	291.5	334.5	344.0
Deferred Policy Acquisition Costs, Amortization Expense	(341.0)	(344.0)	(315.1)
Adjustment Related to Unrealized Investment Gains and Losses	(4.8)	(6.9)	5.1
Deferred Policy Acquisition Costs Foreign Currency Translation Adjustment And Other	0.0	0.0	0.0
Deferred Acquisition Costs at End of Year	1,168.7	1,223.0	1,239.4
Operating Segments | Unum International
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	26.4	20.0	21.3
Deferral of Acquisition Costs	12.1	12.8	8.1
Deferred Policy Acquisition Costs, Amortization Expense	(7.4)	(7.1)	(8.2)
Adjustment Related to Unrealized Investment Gains and Losses	0.0	0.0	0.0
Deferred Policy Acquisition Costs Foreign Currency Translation Adjustment And Other	0.9	0.7	(1.2)
Deferred Acquisition Costs at End of Year	32.0	26.4	20.0
Operating Segments | Colonial Life
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	1,074.6	1,050.0	957.9
Deferral of Acquisition Costs	272.6	311.3	315.9
Deferred Policy Acquisition Costs, Amortization Expense	(257.7)	(258.8)	(242.2)
Adjustment Related to Unrealized Investment Gains and Losses	(17.6)	(27.9)	18.4
Deferred Policy Acquisition Costs Foreign Currency Translation Adjustment And Other	0.0	0.0	0.0
Deferred Acquisition Costs at End of Year	$ 1,071.9	$ 1,074.6	$ 1,050.0