Assets by Segment (Detail) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Segment Reporting Information
Assets	$ 66,955.7	$ 61,875.6
Operating Segments
Segment Reporting Information
Assets	66,955.7	61,875.6
Operating Segments | Unum US
Segment Reporting Information
Assets	18,617.1	17,510.9
Operating Segments | Unum International
Segment Reporting Information
Assets	3,713.5	3,426.8
Operating Segments | Colonial Life
Segment Reporting Information
Assets	4,608.2	4,237.9
Operating Segments | Closed Block
Segment Reporting Information
Assets	36,906.3	34,527.6
Operating Segments | Corporate and Other
Segment Reporting Information
Assets	$ 3,110.6	$ 2,172.4