Assets by Segment (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Assets	$ 59,585.8	$ 62,236.1
Unum U.S.
Segment Reporting Information [Line Items]
Assets	18,569.7	19,391.2
Unum U.K.
Segment Reporting Information [Line Items]
Assets	3,690.1	3,975.8
Colonial Life
Segment Reporting Information [Line Items]
Assets	3,435.4	3,434.9
Closed Block
Segment Reporting Information [Line Items]
Assets	31,651.2	33,069.2
Corporate
Segment Reporting Information [Line Items]
Assets	$ 2,239.4	$ 2,365.0