Segment Information Deferred Acquisition Costs by Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	$ 2,008.5
Deferred Policy Acquisition Cost, Amortization Expense	(493.0)	$ (482.3)	$ (440.8)
Deferred Acquisition Costs at End of Year	2,094.2	2,008.5
Operating Segments
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	2,008.5	1,901.3	1,829.2
Deferral of Acquisition Costs	592.4	569.7	524.0
Deferred Policy Acquisition Cost, Amortization Expense	(493.0)	(482.3)	(440.8)
Adjustment Related to Unrealized Investment Gains and Losses	(9.5)	21.4	(9.2)
Deferred Policy Acquisition Costs Foreign Currency Translation Adjustment And Other	(4.2)	(1.6)	(1.9)
Deferred Acquisition Costs at End of Year	2,094.2	2,008.5	1,901.3
Operating Segments | Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	1,136.4	1,096.5	1,051.5
Deferral of Acquisition Costs	314.1	307.3	292.7
Deferred Policy Acquisition Cost, Amortization Expense	(275.2)	(272.3)	(248.1)
Adjustment Related to Unrealized Investment Gains and Losses	1.2	4.9	0.4
Deferred Policy Acquisition Costs Foreign Currency Translation Adjustment And Other	0.0	0.0	0.0
Deferred Acquisition Costs at End of Year	1,176.5	1,136.4	1,096.5
Operating Segments | Unum UK
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	27.1	30.4	34.3
Deferral of Acquisition Costs	8.2	9.6	10.5
Deferred Policy Acquisition Cost, Amortization Expense	(9.7)	(11.3)	(12.5)
Adjustment Related to Unrealized Investment Gains and Losses	0.0	0.0	0.0
Deferred Policy Acquisition Costs Foreign Currency Translation Adjustment And Other	(4.2)	(1.6)	(1.9)
Deferred Acquisition Costs at End of Year	21.4	27.1	30.4
Operating Segments | Colonial Life
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	845.0	774.4	743.4
Deferral of Acquisition Costs	270.1	252.8	220.8
Deferred Policy Acquisition Cost, Amortization Expense	(208.1)	(198.7)	(180.2)
Adjustment Related to Unrealized Investment Gains and Losses	(10.7)	16.5	(9.6)
Deferred Policy Acquisition Costs Foreign Currency Translation Adjustment And Other	0.0	0.0	0.0
Deferred Acquisition Costs at End of Year	$ 896.3	$ 845.0	$ 774.4