Premium Income by Major Line of Business within Each Segment (Detail) (USD $) In Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Segment Reporting Information [Line Items]
Premiums Earned, Net	$ 1,875.0	$ 1,849.8	$ 3,744.5	$ 3,713.0
Unum U.S.
Segment Reporting Information [Line Items]
Premiums Earned, Net	1,220.6	1,216.5	2,440.0	2,432.6
Unum U.S. | Group Long term Disability
Segment Reporting Information [Line Items]
Premiums Earned, Net	394.6	413.1	791.6	828.7
Unum U.S. | Group Short-term Disability
Segment Reporting Information [Line Items]
Premiums Earned, Net	112.8	107.6	223.8	213.8
Unum U.S. | Group Life
Segment Reporting Information [Line Items]
Premiums Earned, Net	274.3	271.4	547.7	541.5
Unum U.S. | Accidental Death & Dismemberment
Segment Reporting Information [Line Items]
Premiums Earned, Net	26.8	26.3	53.9	52.3
Unum U.S. | Individual Disability-Recently Issued
Segment Reporting Information [Line Items]
Premiums Earned, Net	116.6	114.4	231.7	232.8
Unum U.S. | Long-term Care
Segment Reporting Information [Line Items]
Premiums Earned, Net	150.8	149.5	301.4	298.7
Unum U.S. | Voluntary Benefits
Segment Reporting Information [Line Items]
Premiums Earned, Net	144.7	134.2	289.9	264.8
Unum U.K.
Segment Reporting Information [Line Items]
Premiums Earned, Net	175.8	152.9	342.9	318.7
Unum U.K. | Group Long term Disability UK [Member]
Segment Reporting Information [Line Items]
Premiums Earned, Net	107.6	99.6	211.0	208.9
Unum U.K. | Group Life UK [Member]
Segment Reporting Information [Line Items]
Premiums Earned, Net	51.9	39.5	99.5	82.3
Unum U.K. | Supplemental and Voluntary UK [Member]
Segment Reporting Information [Line Items]
Premiums Earned, Net	16.3	13.8	32.4	27.5
Colonial Life
Segment Reporting Information [Line Items]
Premiums Earned, Net	282.0	267.7	562.4	532.8
Colonial Life | Accident, Sickness, and Disability - Colonial
Segment Reporting Information [Line Items]
Premiums Earned, Net	173.1	164.2	345.4	327.0
Colonial Life | Life Colonial
Segment Reporting Information [Line Items]
Premiums Earned, Net	46.8	44.1	93.0	87.6
Colonial Life | Cancer and Critical Illness - Colonial
Segment Reporting Information [Line Items]
Premiums Earned, Net	62.1	59.4	124.0	118.2
Individual Disability-Closed Block
Segment Reporting Information [Line Items]
Premiums Earned, Net	196.3	212.5	398.6	426.5
Corporate and Other
Segment Reporting Information [Line Items]
Premiums Earned, Net	$ 0.3	$ 0.2	$ 0.6	$ 2.4