Other Income Tax - Additional Information (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2017
State and Local Jurisdiction
Operating Loss Carryforwards
Operating Loss Carryforwards	$ 156.4	$ 155.4
Deferred Tax Assets, Operating Loss Carryforwards	16.0	18.0
Operating Loss Carryforwards, Valuation Allowance	14.1	$ 15.7
Unum UK
Operating Loss Carryforwards
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount	$ 4.5