Premium Income by Major Line of Business within Each Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Segment Reporting Information [Line Items]
Premiums Earned, Net	$ 1,930.9	$ 1,921.9
Unum U.S.
Segment Reporting Information [Line Items]
Premiums Earned, Net	1,139.7	1,112.0
Unum U.S. | Group Long term Disability
Segment Reporting Information [Line Items]
Premiums Earned, Net	392.7	395.5
Unum U.S. | Group Short term Disability [Member]
Segment Reporting Information [Line Items]
Premiums Earned, Net	131.0	117.7
Unum U.S. | Group Life [Member]
Segment Reporting Information [Line Items]
Premiums Earned, Net	302.2	293.6
Unum U.S. | Accidental Death & Dismemberment
Segment Reporting Information [Line Items]
Premiums Earned, Net	30.9	28.3
Unum U.S. | Individual Disability-Recently Issued
Segment Reporting Information [Line Items]
Premiums Earned, Net	120.1	118.4
Unum U.S. | Voluntary Benefits
Segment Reporting Information [Line Items]
Premiums Earned, Net	162.8	158.5
Unum U.K.
Segment Reporting Information [Line Items]
Premiums Earned, Net	143.8	170.7
Unum U.K. | Group Long term Disability UK
Segment Reporting Information [Line Items]
Premiums Earned, Net	97.9	101.9
Unum U.K. | Group Life UK
Segment Reporting Information [Line Items]
Premiums Earned, Net	31.0	52.7
Unum U.K. | Supplemental and Voluntary UK
Segment Reporting Information [Line Items]
Premiums Earned, Net	14.9	16.1
Colonial Life
Segment Reporting Information [Line Items]
Premiums Earned, Net	307.1	296.3
Colonial Life | Accident, Sickness, and Disability - Colonial
Segment Reporting Information [Line Items]
Premiums Earned, Net	184.8	180.2
Colonial Life | Life Colonial [Member]
Segment Reporting Information [Line Items]
Premiums Earned, Net	54.8	51.7
Colonial Life | Cancer and Critical Illness - Colonial
Segment Reporting Information [Line Items]
Premiums Earned, Net	67.5	64.4
Closed Block
Segment Reporting Information [Line Items]
Premiums Earned, Net	340.3	342.9
Closed Block | Individual Disability Closed Block
Segment Reporting Information [Line Items]
Premiums Earned, Net	181.8	187.2
Closed Block | Long-term Care
Segment Reporting Information [Line Items]
Premiums Earned, Net	158.8	155.2
Closed Block | All Other Segments
Segment Reporting Information [Line Items]
Premiums Earned, Net	$ (0.3)	$ 0.5