Assets by Segment (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Segment Reporting Information
Assets	$ 61,960.6	$ 59,403.6
Operating Segments
Segment Reporting Information
Assets	61,960.6	59,403.6
Operating Segments | Unum US
Segment Reporting Information
Assets	18,672.5	18,384.3
Operating Segments | Unum UK
Segment Reporting Information
Assets	3,798.0	3,654.1
Operating Segments | Colonial Life
Segment Reporting Information
Assets	3,623.9	3,482.9
Operating Segments | Closed Block
Segment Reporting Information
Assets	33,151.7	31,564.2
Operating Segments | Corporate and Other
Segment Reporting Information
Assets	$ 2,714.5	$ 2,318.1