Premium Income by Major Line of Business within Each Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Segment Reporting Information [Line Items]
Premiums Earned, Net	$ 1,921.9	$ 1,869.5
Unum U.S.
Segment Reporting Information [Line Items]
Premiums Earned, Net	1,112.0	1,068.8
Unum U.S. | Group Long term Disability
Segment Reporting Information [Line Items]
Premiums Earned, Net	395.5	397.0
Unum U.S. | Group Short-term Disability
Segment Reporting Information [Line Items]
Premiums Earned, Net	117.7	111.0
Unum U.S. | Group Life
Segment Reporting Information [Line Items]
Premiums Earned, Net	293.6	273.4
Unum U.S. | Accidental Death & Dismemberment
Segment Reporting Information [Line Items]
Premiums Earned, Net	28.3	27.1
Unum U.S. | Individual Disability-Recently Issued
Segment Reporting Information [Line Items]
Premiums Earned, Net	118.4	115.1
Unum U.S. | Voluntary Benefits
Segment Reporting Information [Line Items]
Premiums Earned, Net	158.5	145.2
Unum U.K.
Segment Reporting Information [Line Items]
Premiums Earned, Net	170.7	167.1
Unum U.K. | Group Long term Disability UK
Segment Reporting Information [Line Items]
Premiums Earned, Net	101.9	103.4
Unum U.K. | Group Life UK
Segment Reporting Information [Line Items]
Premiums Earned, Net	52.7	47.6
Unum U.K. | Supplemental and Voluntary UK
Segment Reporting Information [Line Items]
Premiums Earned, Net	16.1	16.1
Colonial Life
Segment Reporting Information [Line Items]
Premiums Earned, Net	296.3	280.4
Colonial Life | Accident, Sickness, and Disability - Colonial
Segment Reporting Information [Line Items]
Premiums Earned, Net	180.2	172.3
Colonial Life | Life Colonial
Segment Reporting Information [Line Items]
Premiums Earned, Net	51.7	46.2
Colonial Life | Cancer and Critical Illness - Colonial
Segment Reporting Information [Line Items]
Premiums Earned, Net	64.4	61.9
Closed Block
Segment Reporting Information [Line Items]
Premiums Earned, Net	342.9	353.2
Closed Block | Individual Disability Closed Block
Segment Reporting Information [Line Items]
Premiums Earned, Net	187.2	202.3
Closed Block | Long-term Care
Segment Reporting Information [Line Items]
Premiums Earned, Net	155.2	150.6
Closed Block | All Other Segments
Segment Reporting Information [Line Items]
Premiums Earned, Net	$ 0.5	$ 0.3