Assets by Segment (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Assets	$ 61,611.4	$ 62,236.1
Unum U.S.
Segment Reporting Information [Line Items]
Assets	19,097.5	19,391.2
Unum U.K.
Segment Reporting Information [Line Items]
Assets	3,664.8	3,975.8
Colonial Life
Segment Reporting Information [Line Items]
Assets	3,438.8	3,434.9
Closed Block
Segment Reporting Information [Line Items]
Assets	32,885.5	33,069.2
Corporate
Segment Reporting Information [Line Items]
Assets	$ 2,524.8	$ 2,365.0