Premium Income by Major Line of Business within Each Segment (Detail) (USD $) In Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Segment Reporting Information [Line Items]
Premiums Earned, Net	$ 1,881.2	$ 1,850.2	$ 5,625.7	$ 5,563.2
Unum U.S.
Segment Reporting Information [Line Items]
Premiums Earned, Net	1,226.5	1,210.6	3,666.5	3,643.2
Unum U.S. | Group Long term Disability
Segment Reporting Information [Line Items]
Premiums Earned, Net	393.7	404.9	1,185.3	1,233.6
Unum U.S. | Group Short-term Disability
Segment Reporting Information [Line Items]
Premiums Earned, Net	115.2	108.7	339.0	322.5
Unum U.S. | Group Life
Segment Reporting Information [Line Items]
Premiums Earned, Net	277.6	273.0	825.3	814.5
Unum U.S. | Accidental Death & Dismemberment
Segment Reporting Information [Line Items]
Premiums Earned, Net	27.7	27.0	81.6	79.3
Unum U.S. | Individual Disability-Recently Issued
Segment Reporting Information [Line Items]
Premiums Earned, Net	116.6	116.7	348.3	349.5
Unum U.S. | Long-term Care
Segment Reporting Information [Line Items]
Premiums Earned, Net	152.1	149.4	453.5	448.1
Unum U.S. | Voluntary Benefits
Segment Reporting Information [Line Items]
Premiums Earned, Net	143.6	130.9	433.5	395.7
Unum U.K.
Segment Reporting Information [Line Items]
Premiums Earned, Net	175.5	161.4	518.4	480.1
Unum U.K. | Group Long term Disability UK [Member]
Segment Reporting Information [Line Items]
Premiums Earned, Net	107.3	101.8	318.3	310.7
Unum U.K. | Group Life UK [Member]
Segment Reporting Information [Line Items]
Premiums Earned, Net	52.1	44.5	151.6	126.8
Unum U.K. | Supplemental and Voluntary UK [Member]
Segment Reporting Information [Line Items]
Premiums Earned, Net	16.1	15.1	48.5	42.6
Colonial Life
Segment Reporting Information [Line Items]
Premiums Earned, Net	283.7	269.3	846.1	802.1
Colonial Life | Accident, Sickness, and Disability - Colonial
Segment Reporting Information [Line Items]
Premiums Earned, Net	174.3	166.1	519.7	493.1
Colonial Life | Life Colonial
Segment Reporting Information [Line Items]
Premiums Earned, Net	47.0	43.4	140.0	131.0
Colonial Life | Cancer and Critical Illness - Colonial
Segment Reporting Information [Line Items]
Premiums Earned, Net	62.4	59.8	186.4	178.0
Individual Disability-Closed Block
Segment Reporting Information [Line Items]
Premiums Earned, Net	196.4	208.5	595.0	635.0
Corporate and Other
Segment Reporting Information [Line Items]
Premiums Earned, Net	$ (0.9)	$ 0.4	$ (0.3)	$ 2.8