Premium Income by Major Line of Business within Each Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Segment Reporting Information [Line Items]
Premiums Earned, Net	$ 1,929.4	$ 1,881.2	$ 5,778.9	$ 5,625.7
Unum U.S.
Segment Reporting Information [Line Items]
Premiums Earned, Net	1,111.9	1,074.4	3,338.9	3,213.0
Unum U.S. | Group Long term Disability
Segment Reporting Information [Line Items]
Premiums Earned, Net	392.4	393.7	1,185.7	1,185.3
Unum U.S. | Group Short-term Disability
Segment Reporting Information [Line Items]
Premiums Earned, Net	120.1	115.2	356.0	339.0
Unum U.S. | Group Life
Segment Reporting Information [Line Items]
Premiums Earned, Net	297.3	277.6	885.1	825.3
Unum U.S. | Accidental Death & Dismemberment
Segment Reporting Information [Line Items]
Premiums Earned, Net	29.2	27.7	86.2	81.6
Unum U.S. | Individual Disability-Recently Issued
Segment Reporting Information [Line Items]
Premiums Earned, Net	119.9	116.6	357.0	348.3
Unum U.S. | Voluntary Benefits
Segment Reporting Information [Line Items]
Premiums Earned, Net	153.0	143.6	468.9	433.5
Unum U.K.
Segment Reporting Information [Line Items]
Premiums Earned, Net	175.2	175.5	519.1	518.4
Unum U.K. | Group Long term Disability UK
Segment Reporting Information [Line Items]
Premiums Earned, Net	102.9	107.3	306.5	318.3
Unum U.K. | Group Life UK
Segment Reporting Information [Line Items]
Premiums Earned, Net	56.4	52.1	164.6	151.6
Unum U.K. | Supplemental and Voluntary UK
Segment Reporting Information [Line Items]
Premiums Earned, Net	15.9	16.1	48.0	48.5
Colonial Life
Segment Reporting Information [Line Items]
Premiums Earned, Net	299.4	283.7	892.6	846.1
Colonial Life | Accident, Sickness, and Disability - Colonial
Segment Reporting Information [Line Items]
Premiums Earned, Net	181.8	174.3	541.8	519.7
Colonial Life | Life Colonial
Segment Reporting Information [Line Items]
Premiums Earned, Net	52.2	47.0	156.3	140.0
Colonial Life | Cancer and Critical Illness - Colonial
Segment Reporting Information [Line Items]
Premiums Earned, Net	65.4	62.4	194.5	186.4
Closed Block
Segment Reporting Information [Line Items]
Premiums Earned, Net	342.9	347.6	1,028.3	1,048.2
Closed Block | Individual Disability Closed Block
Segment Reporting Information [Line Items]
Premiums Earned, Net	183.1	196.4	554.9	595.0
Closed Block | Long-term Care
Segment Reporting Information [Line Items]
Premiums Earned, Net	159.4	152.1	471.5	453.5
Closed Block | All Other Segments
Segment Reporting Information [Line Items]
Premiums Earned, Net	$ 0.4	$ (0.9)	$ 1.9	$ (0.3)