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, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Segment Reporting Information [Line Items]
Premiums Earned, Net	$ 1,897.3	$ 1,929.4	$ 5,734.0	$ 5,778.9
Unum U.S.
Segment Reporting Information [Line Items]
Premiums Earned, Net	1,124.6	1,111.9	3,395.8	3,338.9
Unum U.S. | Group Long-term Disability - US
Segment Reporting Information [Line Items]
Premiums Earned, Net	385.0	392.4	1,172.5	1,185.7
Unum U.S. | Group Short-term Disability - US
Segment Reporting Information [Line Items]
Premiums Earned, Net	129.5	120.1	389.6	356.0
Unum U.S. | Group Life - US
Segment Reporting Information [Line Items]
Premiums Earned, Net	305.2	297.3	911.0	885.1
Unum U.S. | Accidental Death & Dismemberment - US
Segment Reporting Information [Line Items]
Premiums Earned, Net	30.5	29.2	91.6	86.2
Unum U.S. | Individual Disability-Recently Issued - US
Segment Reporting Information [Line Items]
Premiums Earned, Net	114.9	119.9	349.3	357.0
Unum U.S. | Voluntary Benefits - US
Segment Reporting Information [Line Items]
Premiums Earned, Net	159.5	153.0	481.8	468.9
Unum U.K.
Segment Reporting Information [Line Items]
Premiums Earned, Net	137.3	175.2	418.7	519.1
Unum U.K. | Group Long term Disability - UK
Segment Reporting Information [Line Items]
Premiums Earned, Net	96.5	102.9	290.7	306.5
Unum U.K. | Group Life - UK
Segment Reporting Information [Line Items]
Premiums Earned, Net	25.6	56.4	83.1	164.6
Unum U.K. | Supplemental and Voluntary - UK
Segment Reporting Information [Line Items]
Premiums Earned, Net	15.2	15.9	44.9	48.0
Colonial Life
Segment Reporting Information [Line Items]
Premiums Earned, Net	309.1	299.4	924.1	892.6
Colonial Life | Accident, Sickness, and Disability - Colonial
Segment Reporting Information [Line Items]
Premiums Earned, Net	185.0	181.8	554.5	541.8
Colonial Life | Life - Colonial
Segment Reporting Information [Line Items]
Premiums Earned, Net	55.6	52.2	165.7	156.3
Colonial Life | Cancer and Critical Illness - Colonial
Segment Reporting Information [Line Items]
Premiums Earned, Net	68.5	65.4	203.9	194.5
Closed Block
Segment Reporting Information [Line Items]
Premiums Earned, Net	326.3	342.9	995.4	1,028.3
Closed Block | Individual Disability - Closed Block
Segment Reporting Information [Line Items]
Premiums Earned, Net	169.3	183.1	521.7	554.9
Closed Block | Long-term Care - Closed Block
Segment Reporting Information [Line Items]
Premiums Earned, Net	157.0	159.4	473.6	471.5
Closed Block | All Other - Closed Block
Segment Reporting Information [Line Items]
Premiums Earned, Net	$ 0	$ 0.4	$ 0.1	$ 1.9