Liability for Unpaid Claims - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability for Unpaid Claims and CAE Narrative Information
Unclaimed Death Benefit Reserve Increase Before Taxes	$ 39.0
Operating Segments
Liability for Unpaid Claims and CAE Narrative Information
Unclaimed Death Benefit Reserve Increase Before Taxes	39.0	$ 0.0	$ 0.0
Operating Segments | Unum US | Group Life
Liability for Unpaid Claims and CAE Narrative Information
Unclaimed Death Benefit Reserve Increase Before Taxes	18.5
Operating Segments | Unum US | Voluntary Benefits
Liability for Unpaid Claims and CAE Narrative Information
Unclaimed Death Benefit Reserve Increase Before Taxes	8.1
Operating Segments | Colonial Life | Life Insurance
Liability for Unpaid Claims and CAE Narrative Information
Unclaimed Death Benefit Reserve Increase Before Taxes	$ 12.4