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, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Segment Reporting Information
Premium Income	$ 1,943.6	$ 1,905.8	$ 3,882.1	$ 3,836.7
Unum US
Segment Reporting Information
Premium Income	1,157.7	1,131.5	2,310.2	2,271.2
Unum US | Group Long term Disability
Segment Reporting Information
Premium Income	386.2	394.8	769.7	787.5
Unum US | Group Short term Disability
Segment Reporting Information
Premium Income	137.8	129.1	275.1	260.1
Unum US | Group Life
Segment Reporting Information
Premium Income	313.2	303.6	623.1	605.8
Unum US | Accidental Death Dismemberment
Segment Reporting Information
Premium Income	31.1	30.2	61.7	61.1
Unum US | Individual Disability Recently Issued
Segment Reporting Information
Premium Income	116.0	114.3	231.9	234.4
Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	173.4	159.5	348.7	322.3
Unum UK
Segment Reporting Information
Premium Income	154.0	137.6	305.7	281.4
Unum UK | Group Long term Disability
Segment Reporting Information
Premium Income	105.9	96.3	208.9	194.2
Unum UK | Group Life
Segment Reporting Information
Premium Income	33.7	26.5	68.5	57.5
Unum UK | Supplemental Insurance
Segment Reporting Information
Premium Income	14.4	14.8	28.3	29.7
Colonial Life
Segment Reporting Information
Premium Income	316.8	307.9	633.2	615.0
Colonial Life | Accident, Sickness, and Disability Insurance
Segment Reporting Information
Premium Income	189.2	184.7	378.3	369.5
Colonial Life | Life Insurance Product Line
Segment Reporting Information
Premium Income	57.5	55.3	114.9	110.1
Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income	70.1	67.9	140.0	135.4
Closed Block
Segment Reporting Information
Premium Income	315.1	328.8	633.0	669.1
Closed Block | Individual Disability Insurance
Segment Reporting Information
Premium Income	157.8	170.6	318.9	352.4
Closed Block | Long-term Care
Segment Reporting Information
Premium Income	157.1	157.8	313.6	316.6
Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 0.2	$ 0.4	$ 0.5	$ 0.1