Premium Income by Major Line of Business within Each Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Segment Reporting Information [Line Items]
Premiums Earned, Net	$ 1,905.8	$ 1,927.6	$ 3,836.7	$ 3,849.5
Unum U.S.
Segment Reporting Information [Line Items]
Premiums Earned, Net	1,131.5	1,115.0	2,271.2	2,227.0
Unum U.S. | Group Long-term Disability - US
Segment Reporting Information [Line Items]
Premiums Earned, Net	394.8	397.8	787.5	793.3
Unum U.S. | Group Short-term Disability - US
Segment Reporting Information [Line Items]
Premiums Earned, Net	129.1	118.2	260.1	235.9
Unum U.S. | Group Life - US
Segment Reporting Information [Line Items]
Premiums Earned, Net	303.6	294.2	605.8	587.8
Unum U.S. | Accidental Death & Dismemberment - US
Segment Reporting Information [Line Items]
Premiums Earned, Net	30.2	28.7	61.1	57.0
Unum U.S. | Individual Disability-Recently Issued - US
Segment Reporting Information [Line Items]
Premiums Earned, Net	114.3	118.7	234.4	237.1
Unum U.S. | Voluntary Benefits - US
Segment Reporting Information [Line Items]
Premiums Earned, Net	159.5	157.4	322.3	315.9
Unum U.K.
Segment Reporting Information [Line Items]
Premiums Earned, Net	137.6	173.2	281.4	343.9
Unum U.K. | Group Long term Disability - UK
Segment Reporting Information [Line Items]
Premiums Earned, Net	96.3	101.7	194.2	203.6
Unum U.K. | Group Life - UK
Segment Reporting Information [Line Items]
Premiums Earned, Net	26.5	55.5	57.5	108.2
Unum U.K. | Supplemental and Voluntary - UK
Segment Reporting Information [Line Items]
Premiums Earned, Net	14.8	16.0	29.7	32.1
Colonial Life
Segment Reporting Information [Line Items]
Premiums Earned, Net	307.9	296.9	615.0	593.2
Colonial Life | Accident, Sickness, and Disability - Colonial
Segment Reporting Information [Line Items]
Premiums Earned, Net	184.7	179.8	369.5	360.0
Colonial Life | Life - Colonial
Segment Reporting Information [Line Items]
Premiums Earned, Net	55.3	52.4	110.1	104.1
Colonial Life | Cancer and Critical Illness - Colonial
Segment Reporting Information [Line Items]
Premiums Earned, Net	67.9	64.7	135.4	129.1
Closed Block
Segment Reporting Information [Line Items]
Premiums Earned, Net	328.8	342.5	669.1	685.4
Closed Block | Individual Disability - Closed Block
Segment Reporting Information [Line Items]
Premiums Earned, Net	170.6	184.6	352.4	371.8
Closed Block | Long-term Care - Closed Block
Segment Reporting Information [Line Items]
Premiums Earned, Net	157.8	156.9	316.6	312.1
Closed Block | All Other - Closed Block
Segment Reporting Information [Line Items]
Premiums Earned, Net	$ 0.4	$ 1.0	$ 0.1	$ 1.5