Assets by Segment (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Segment Reporting Information [Line Items]
Assets	$ 59,571.8	$ 59,555.2
Unum U.S.
Segment Reporting Information [Line Items]
Assets	18,621.5	18,583.6
Unum U.K.
Segment Reporting Information [Line Items]
Assets	3,746.8	3,549.5
Colonial Life
Segment Reporting Information [Line Items]
Assets	3,209.3	3,167.8
Closed Block
Segment Reporting Information [Line Items]
Assets	31,306.2	31,439.5
Corporate
Segment Reporting Information [Line Items]
Assets	$ 2,688.0	$ 2,814.8