Premium Income by Major Line of Business within Each Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Segment Reporting Information [Line Items]
Premium Income	$ 1,938.5	$ 1,930.9
Unum U.S.
Segment Reporting Information [Line Items]
Premium Income	1,152.5	1,139.7
Unum U.K.
Segment Reporting Information [Line Items]
Premium Income	151.7	143.8
Colonial Life
Segment Reporting Information [Line Items]
Premium Income	316.4	307.1
Closed Block
Segment Reporting Information [Line Items]
Premium Income	317.9	340.3
Group Long term Disability | Unum U.S.
Segment Reporting Information [Line Items]
Premium Income	383.5	392.7
Group Long term Disability | Unum U.K.
Segment Reporting Information [Line Items]
Premium Income	103.0	97.9
Group Short term Disability | Unum U.S.
Segment Reporting Information [Line Items]
Premium Income	137.3	131.0
Group Life | Unum U.S.
Segment Reporting Information [Line Items]
Premium Income	309.9	302.2
Group Life | Unum U.K.
Segment Reporting Information [Line Items]
Premium Income	34.8	31.0
Accidental Death Dismemberment | Unum U.S.
Segment Reporting Information [Line Items]
Premium Income	30.6	30.9
Individual Disability Recently Issued | Unum U.S.
Segment Reporting Information [Line Items]
Premium Income	115.9	120.1
Voluntary Benefits | Unum U.S.
Segment Reporting Information [Line Items]
Premium Income	175.3	162.8
Supplemental and Voluntary Insurance | Unum U.K.
Segment Reporting Information [Line Items]
Premium Income	13.9	14.9
Accident and Health Insurance Segment | Colonial Life
Segment Reporting Information [Line Items]
Premium Income	189.1	184.8
Life Insurance Product Line | Colonial Life
Segment Reporting Information [Line Items]
Premium Income	57.4	54.8
Cancer and Critical Illness Colonial | Colonial Life
Segment Reporting Information [Line Items]
Premium Income	69.9	67.5
Individual Disability Insurance | Closed Block
Segment Reporting Information [Line Items]
Premium Income	161.1	181.8
Long-term Care | Closed Block
Segment Reporting Information [Line Items]
Premium Income	156.5	158.8
Other Insurance Product Line | Closed Block
Segment Reporting Information [Line Items]
Premium Income	$ 0.3	$ (0.3)