Liability for Unpaid Claims - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Liability for Unpaid Claims and CAE Narrative Information
Long-term Care Reserve Increase, Before Tax	$ 750.8
Long-term Care Reserve Increase, After Tax	593.1
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	0.0	39.0	$ 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
Claim Reserve
Liability for Unpaid Claims and CAE Narrative Information
Long-term Care Reserve Increase, Before Tax	$ 236.0