Liability for Unpaid Claims - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2014	Dec. 31, 2013
Liability for Unpaid Claims and CAE Narrative Information
Reserve Increase for Long-term Care	$ 698.2
Unclaimed Death Benefit Reserve Increase (Decrease) Before Taxes			$ 95.5
Group Life
Liability for Unpaid Claims and CAE Narrative Information
Unclaimed Death Benefit Reserve Increase (Decrease) Before Taxes			49.1
Unum US Voluntary Life
Liability for Unpaid Claims and CAE Narrative Information
Unclaimed Death Benefit Reserve Increase (Decrease) Before Taxes			26.3
Unum U.S.
Liability for Unpaid Claims and CAE Narrative Information
Unclaimed Death Benefit Reserve Increase (Decrease) Before Taxes			75.4
Unum U.S. | Group Life
Liability for Unpaid Claims and CAE Narrative Information
Waiver of Premium Benefit Reserve Decrease Before Tax			85.0
Claim Reserve Prior Year
Liability for Unpaid Claims and CAE Narrative Information
Reserve Increase for Long-term Care		$ 85.8
Claim Reserve Prior Year | Unum U.S. | Group Life
Liability for Unpaid Claims and CAE Narrative Information
Waiver of Premium Benefit Reserve Decrease Before Tax			$ 78.0