Assets by Segment (Detail) (USD $) In Millions	Jun. 30, 2011	Dec. 31, 2010
Segment Reporting Information [Line Items]
Assets	$ 58,015.8	$ 57,307.7
Unum U.S.
Segment Reporting Information [Line Items]
Assets	25,426.8	24,876.9
Unum U.K.
Segment Reporting Information [Line Items]
Assets	3,583.7	3,386.3
Colonial Life
Segment Reporting Information [Line Items]
Assets	3,190.2	3,047.3
Individual Disability-Closed Block
Segment Reporting Information [Line Items]
Assets	15,472.7	15,509.1
Corporate and Other
Segment Reporting Information [Line Items]
Assets	$ 10,342.4	$ 10,488.1