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, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Segment Reporting Information [Line Items]
Premiums Earned, Net	$ 1,937.2	$ 1,929.4	$ 1,927.6	$ 1,921.9	$ 1,888.5	$ 1,881.2	$ 1,875.0	$ 1,869.5	$ 7,716.1	$ 7,514.2	$ 7,431.4
Unum U.S.
Segment Reporting Information [Line Items]
Premiums Earned, Net									4,456.5	4,296.0	4,255.4
Unum U.S. | Group Long term Disability
Segment Reporting Information [Line Items]
Premiums Earned, Net									1,578.8	1,580.2	1,639.4
Unum U.S. | Group Short-term Disability
Segment Reporting Information [Line Items]
Premiums Earned, Net									476.7	455.2	430.9
Unum U.S. | Group Life
Segment Reporting Information [Line Items]
Premiums Earned, Net									1,182.1	1,106.7	1,090.3
Unum U.S. | Accidental Death & Dismemberment
Segment Reporting Information [Line Items]
Premiums Earned, Net									115.3	109.2	106.1
Unum U.S. | Individual Disability-Recently Issued
Segment Reporting Information [Line Items]
Premiums Earned, Net									477.6	464.7	457.9
Unum U.S. | Voluntary Benefits
Segment Reporting Information [Line Items]
Premiums Earned, Net									626.0	580.0	530.8
Unum U.K.
Segment Reporting Information [Line Items]
Premiums Earned, Net									694.6	687.6	650.6
Unum U.K. | Group Long term Disability UK
Segment Reporting Information [Line Items]
Premiums Earned, Net									409.7	419.6	421.2
Unum U.K. | Group Life UK
Segment Reporting Information [Line Items]
Premiums Earned, Net									221.3	203.6	171.6
Unum U.K. | Supplemental and Voluntary UK
Segment Reporting Information [Line Items]
Premiums Earned, Net									63.6	64.4	57.8
Colonial Life
Segment Reporting Information [Line Items]
Premiums Earned, Net									1,194.5	1,135.3	1,075.7
Colonial Life | Accident, Sickness, and Disability - Colonial
Segment Reporting Information [Line Items]
Premiums Earned, Net									724.5	695.3	661.0
Colonial Life | Life Colonial [Member]
Segment Reporting Information [Line Items]
Premiums Earned, Net									209.7	190.7	176.5
Colonial Life | Cancer and Critical Illness - Colonial
Segment Reporting Information [Line Items]
Premiums Earned, Net									260.3	249.3	238.2
Closed Block
Segment Reporting Information [Line Items]
Premiums Earned, Net									1,370.5	1,395.3	1,449.7
Closed Block | Individual Disability Closed Block
Segment Reporting Information [Line Items]
Premiums Earned, Net									736.4	787.0	847.0
Closed Block | Long-term Care
Segment Reporting Information [Line Items]
Premiums Earned, Net									631.9	608.1	599.2
Closed Block | All Other Segments
Segment Reporting Information [Line Items]
Premiums Earned, Net									$ 2.2	$ 0.2	$ 3.5