Premium Income by Major Line of Business within Each Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Segment Reporting Information [Line Items]
Premiums Earned, Net	$ 1,888.5	$ 1,881.2	$ 1,875.0	$ 1,869.5	$ 1,868.2	$ 1,850.2	$ 1,849.8	$ 1,863.2	$ 7,514.2	$ 7,431.4	$ 7,475.5
Unum U.S.
Segment Reporting Information [Line Items]
Premiums Earned, Net									4,296.0	4,255.4	4,278.4
Unum U.S. | Group Long term Disability
Segment Reporting Information [Line Items]
Premiums Earned, Net									1,580.2	1,639.4	1,726.9
Unum U.S. | Group Short-term Disability
Segment Reporting Information [Line Items]
Premiums Earned, Net									455.2	430.9	432.8
Unum U.S. | Group Life
Segment Reporting Information [Line Items]
Premiums Earned, Net									1,106.7	1,090.3	1,057.7
Unum U.S. | Accidental Death & Dismemberment
Segment Reporting Information [Line Items]
Premiums Earned, Net									109.2	106.1	104.9
Unum U.S. | Individual Disability-Recently Issued
Segment Reporting Information [Line Items]
Premiums Earned, Net									464.7	457.9	463.7
Unum U.S. | Voluntary Benefits
Segment Reporting Information [Line Items]
Premiums Earned, Net									580.0	530.8	492.4
Unum U.K.
Segment Reporting Information [Line Items]
Premiums Earned, Net									687.6	650.6	686.1
Unum U.K. | Group Long term Disability UK
Segment Reporting Information [Line Items]
Premiums Earned, Net									419.6	421.2	482.4
Unum U.K. | Group Life UK
Segment Reporting Information [Line Items]
Premiums Earned, Net									203.6	171.6	147.8
Unum U.K. | Supplemental and Voluntary UK
Segment Reporting Information [Line Items]
Premiums Earned, Net									64.4	57.8	55.9
Colonial Life
Segment Reporting Information [Line Items]
Premiums Earned, Net									1,135.3	1,075.7	1,015.1
Colonial Life | Accident, Sickness, and Disability - Colonial
Segment Reporting Information [Line Items]
Premiums Earned, Net									695.3	661.0	625.8
Colonial Life | Life Colonial
Segment Reporting Information [Line Items]
Premiums Earned, Net									190.7	176.5	165.6
Colonial Life | Cancer and Critical Illness - Colonial
Segment Reporting Information [Line Items]
Premiums Earned, Net									249.3	238.2	223.7
Closed Block
Segment Reporting Information [Line Items]
Premiums Earned, Net									1,395.3	1,449.7	1,495.9
Closed Block | Individual Disability Closed Block
Segment Reporting Information [Line Items]
Premiums Earned, Net									787.0	847.0	898.5
Closed Block | Long-term Care
Segment Reporting Information [Line Items]
Premiums Earned, Net									608.1	599.2	594.7
Closed Block | All Other Segments
Segment Reporting Information [Line Items]
Premiums Earned, Net									$ 0.2	$ 3.5	$ 2.7