Assets by Segment (Detail) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Segment Reporting Information
Assets	$ 65,843.7	$ 61,875.6
Operating Segments
Segment Reporting Information
Assets	65,843.7	61,875.6
Operating Segments | Unum US
Segment Reporting Information
Assets	18,438.3	17,510.9
Operating Segments | Unum International
Segment Reporting Information
Assets	3,769.4	3,426.8
Operating Segments | Colonial Life
Segment Reporting Information
Assets	4,531.0	4,237.9
Operating Segments | Closed Block
Segment Reporting Information
Assets	36,196.7	34,527.6
Operating Segments | Corporate and Other
Segment Reporting Information
Assets	$ 2,908.3	$ 2,172.4