Assets by Segment (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Assets	$ 59,366.2	$ 62,236.1
Unum U.S.
Segment Reporting Information [Line Items]
Assets	18,529.5	19,391.2
Unum U.K.
Segment Reporting Information [Line Items]
Assets	3,531.1	3,975.8
Colonial Life
Segment Reporting Information [Line Items]
Assets	3,382.6	3,434.9
Closed Block
Segment Reporting Information [Line Items]
Assets	31,689.7	33,069.2
Corporate
Segment Reporting Information [Line Items]
Assets	$ 2,233.3	$ 2,365.0