Segment Information Deferred Acquisition Costs by Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	$ 2,094.2
Deferred Policy Acquisition Costs, Amortization Expense	(527.1)	$ (493.0)	$ (482.3)
Deferred Acquisition Costs at End of Year	2,184.6	2,094.2
Operating Segments
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	2,094.2	2,008.5	1,901.3
Deferral of Acquisition Costs	628.0	592.4	569.7
Deferred Policy Acquisition Costs, Amortization Expense	(527.1)	(493.0)	(482.3)
Adjustment Related to Unrealized Investment Gains and Losses	(12.5)	(9.5)	21.4
Deferred Policy Acquisition Costs Foreign Currency Translation Adjustment And Other	2.0	(4.2)	(1.6)
Deferred Acquisition Costs at End of Year	2,184.6	2,094.2	2,008.5
Operating Segments | Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	1,176.5	1,136.4	1,096.5
Deferral of Acquisition Costs	325.5	314.1	307.3
Deferred Policy Acquisition Costs, Amortization Expense	(293.6)	(275.2)	(272.3)
Adjustment Related to Unrealized Investment Gains and Losses	(3.0)	1.2	4.9
Deferred Policy Acquisition Costs Foreign Currency Translation Adjustment And Other	0.0	0.0	0.0
Deferred Acquisition Costs at End of Year	1,205.4	1,176.5	1,136.4
Operating Segments | Unum UK
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	21.4	27.1	30.4
Deferral of Acquisition Costs	7.0	8.2	9.6
Deferred Policy Acquisition Costs, Amortization Expense	(9.1)	(9.7)	(11.3)
Adjustment Related to Unrealized Investment Gains and Losses	0.0	0.0	0.0
Deferred Policy Acquisition Costs Foreign Currency Translation Adjustment And Other	2.0	(4.2)	(1.6)
Deferred Acquisition Costs at End of Year	21.3	21.4	27.1
Operating Segments | Colonial Life
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	896.3	845.0	774.4
Deferral of Acquisition Costs	295.5	270.1	252.8
Deferred Policy Acquisition Costs, Amortization Expense	(224.4)	(208.1)	(198.7)
Adjustment Related to Unrealized Investment Gains and Losses	(9.5)	(10.7)	16.5
Deferred Policy Acquisition Costs Foreign Currency Translation Adjustment And Other	0.0	0.0	0.0
Deferred Acquisition Costs at End of Year	$ 957.9	$ 896.3	$ 845.0