Assets by Segment (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Assets	$ 61,078.6	$ 59,403.6
Unum U.S.
Segment Reporting Information [Line Items]
Assets	18,517.9	18,384.3
Unum U.K.
Segment Reporting Information [Line Items]
Assets	3,793.4	3,654.1
Colonial Life
Segment Reporting Information [Line Items]
Assets	3,542.5	3,482.9
Closed Block
Segment Reporting Information [Line Items]
Assets	32,397.3	31,564.2
Corporate
Segment Reporting Information [Line Items]
Assets	$ 2,827.5	$ 2,318.1