Liability for Unpaid Claims - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2014	Dec. 31, 2013
Liability for Unpaid Claims and CAE Narrative Information
Reserve Increase for Long-term Care	$ 698.2		$ 698.2
Unclaimed Death Benefit Reserve Increase (Decrease) Before Taxes		95.5		95.5
Waiver of Premium Benefit Reserve Decrease Before Tax		85.0
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
Colonial Voluntary Life
Liability for Unpaid Claims and CAE Narrative Information
Unclaimed Death Benefit Reserve Increase (Decrease) Before Taxes				20.1
Unum U.S.
Liability for Unpaid Claims and CAE Narrative Information
Unclaimed Death Benefit Reserve Increase (Decrease) Before Taxes				75.4
Waiver of Premium Benefit Reserve Decrease Before Tax				(85.0)
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