Assets by Segment (Detail) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Segment Reporting Information
Assets	$ 63,921.7	$ 61,875.6
Operating Segments
Segment Reporting Information
Assets	63,921.7	61,875.6
Operating Segments | Unum US
Segment Reporting Information
Assets	17,802.1	17,510.9
Operating Segments | Unum International
Segment Reporting Information
Assets	3,646.6	3,426.8
Operating Segments | Colonial Life
Segment Reporting Information
Assets	4,286.8	4,237.9
Operating Segments | Closed Block
Segment Reporting Information
Assets	35,542.2	34,527.6
Operating Segments | Corporate and Other
Segment Reporting Information
Assets	$ 2,644.0	$ 2,172.4