Assets by Segment (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Segment Reporting Information [Line Items]
Assets	$ 60,179.0	$ 57,307.7
Unum U.S.
Segment Reporting Information [Line Items]
Assets	19,004.1	18,572.1
Unum U.K.
Segment Reporting Information [Line Items]
Assets	3,568.1	3,386.3
Colonial Life
Segment Reporting Information [Line Items]
Assets	3,352.5	3,047.3
Closed Block
Segment Reporting Information [Line Items]
Assets	31,439.5	29,418.7
Corporate
Segment Reporting Information [Line Items]
Assets	$ 2,814.8	$ 2,883.3