Assets by Segment (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Segment Reporting Information [Line Items]
Assets	$ 60,678.7	$ 59,555.2
Unum U.S.
Segment Reporting Information [Line Items]
Assets	18,808.3	18,583.6
Unum U.K.
Segment Reporting Information [Line Items]
Assets	3,771.2	3,549.5
Colonial Life
Segment Reporting Information [Line Items]
Assets	3,280.2	3,167.8
Closed Block
Segment Reporting Information [Line Items]
Assets	32,098.1	31,439.5
Corporate
Segment Reporting Information [Line Items]
Assets	$ 2,720.9	$ 2,814.8