Assets by Segment (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Assets	$ 59,403.6	$ 62,236.1
Unum U.S.
Segment Reporting Information [Line Items]
Assets	18,384.3	19,391.2
Unum U.K.
Segment Reporting Information [Line Items]
Assets	3,654.1	3,975.8
Colonial Life
Segment Reporting Information [Line Items]
Assets	3,482.9	3,434.9
Closed Block
Segment Reporting Information [Line Items]
Assets	31,564.2	33,069.2
Corporate
Segment Reporting Information [Line Items]
Assets	$ 2,318.1	$ 2,365.0