Premium Income by Major Line of Business within Each Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Segment Reporting Information [Line Items]
Premiums Earned, Net	$ 1,927.6	$ 1,875.0	$ 3,849.5	$ 3,744.5
Unum U.S.
Segment Reporting Information [Line Items]
Premiums Earned, Net	1,115.0	1,069.8	2,227.0	2,138.6
Unum U.S. | Group Long term Disability
Segment Reporting Information [Line Items]
Premiums Earned, Net	397.8	394.6	793.3	791.6
Unum U.S. | Group Short-term Disability
Segment Reporting Information [Line Items]
Premiums Earned, Net	118.2	112.8	235.9	223.8
Unum U.S. | Group Life
Segment Reporting Information [Line Items]
Premiums Earned, Net	294.2	274.3	587.8	547.7
Unum U.S. | Accidental Death & Dismemberment
Segment Reporting Information [Line Items]
Premiums Earned, Net	28.7	26.8	57.0	53.9
Unum U.S. | Individual Disability-Recently Issued
Segment Reporting Information [Line Items]
Premiums Earned, Net	118.7	116.6	237.1	231.7
Unum U.S. | Voluntary Benefits
Segment Reporting Information [Line Items]
Premiums Earned, Net	157.4	144.7	315.9	289.9
Unum U.K.
Segment Reporting Information [Line Items]
Premiums Earned, Net	173.2	175.8	343.9	342.9
Unum U.K. | Group Long term Disability UK
Segment Reporting Information [Line Items]
Premiums Earned, Net	101.7	107.6	203.6	211.0
Unum U.K. | Group Life UK
Segment Reporting Information [Line Items]
Premiums Earned, Net	55.5	51.9	108.2	99.5
Unum U.K. | Supplemental and Voluntary UK
Segment Reporting Information [Line Items]
Premiums Earned, Net	16.0	16.3	32.1	32.4
Colonial Life
Segment Reporting Information [Line Items]
Premiums Earned, Net	296.9	282.0	593.2	562.4
Colonial Life | Accident, Sickness, and Disability - Colonial
Segment Reporting Information [Line Items]
Premiums Earned, Net	179.8	173.1	360.0	345.4
Colonial Life | Life Colonial
Segment Reporting Information [Line Items]
Premiums Earned, Net	52.4	46.8	104.1	93.0
Colonial Life | Cancer and Critical Illness - Colonial
Segment Reporting Information [Line Items]
Premiums Earned, Net	64.7	62.1	129.1	124.0
Closed Block
Segment Reporting Information [Line Items]
Premiums Earned, Net	342.5	347.4	685.4	700.6
Closed Block | Individual Disability Closed Block
Segment Reporting Information [Line Items]
Premiums Earned, Net	184.6	196.3	371.8	398.6
Closed Block | Long-term Care
Segment Reporting Information [Line Items]
Premiums Earned, Net	156.9	150.8	312.1	301.4
Closed Block | All Other Segments
Segment Reporting Information [Line Items]
Premiums Earned, Net	$ 1.0	$ 0.3	$ 1.5	$ 0.6