Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information
Premium Income	$ 2,158.4	$ 2,153.6	$ 2,142.2	$ 2,142.9	$ 2,099.2	$ 2,089.4	$ 2,081.6	$ 2,087.5	$ 8,597.1	$ 8,357.7	$ 8,082.4
Net Investment Income	$ 619.8	$ 609.0	$ 620.5	$ 602.4	$ 617.9	$ 611.4	$ 623.3	$ 606.4	2,451.7	2,459.0	2,481.2
Other Income									197.7	205.6	211.5
Interest and Debt Expense									159.9	166.0	152.8
Operating Segments
Segment Reporting Information
Premium Income									8,597.1	8,357.7	8,082.4
Net Investment Income									2,451.7	2,459.0	2,481.2
Other Income									197.7	205.6	211.5
Adjusted Operating Revenue									11,246.5	11,022.3	10,775.1
Adjusted Operating Income (Loss)									1,423.3	1,323.5	1,282.1
Interest and Debt Expense									159.9	166.0	152.8
Depreciation and Amortization									636.1	600.2	587.1
Operating Segments | Unum US
Segment Reporting Information
Premium Income									5,443.5	5,240.9	4,960.0
Net Investment Income									811.2	828.7	865.3
Other Income									113.2	113.3	119.2
Adjusted Operating Revenue									6,367.9	6,182.9	5,944.5
Adjusted Operating Income (Loss)									1,009.5	914.2	850.0
Interest and Debt Expense									0.0	0.0	0.0
Depreciation and Amortization									371.3	353.0	347.0
Operating Segments | Unum UK
Segment Reporting Information
Premium Income									513.0	529.3	576.2
Net Investment Income									120.2	118.1	124.9
Other Income									0.7	0.2	0.0
Adjusted Operating Revenue									633.9	647.6	701.1
Adjusted Operating Income (Loss)									111.7	128.6	140.6
Interest and Debt Expense									0.0	0.0	0.0
Depreciation and Amortization									17.3	16.4	18.0
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income									1,511.4	1,417.1	1,338.6
Net Investment Income									144.9	141.5	145.4
Other Income									1.1	1.2	0.1
Adjusted Operating Revenue									1,657.4	1,559.8	1,484.1
Adjusted Operating Income (Loss)									325.0	314.2	309.1
Interest and Debt Expense									0.0	0.0	0.0
Depreciation and Amortization									239.9	222.9	214.6
Operating Segments | Closed Block
Segment Reporting Information
Premium Income									1,129.2	1,170.4	1,207.6
Net Investment Income									1,354.0	1,352.2	1,320.0
Other Income									79.8	86.0	89.3
Adjusted Operating Revenue									2,563.0	2,608.6	2,616.9
Adjusted Operating Income (Loss)									123.9	129.5	119.1
Interest and Debt Expense									6.7	6.9	6.6
Depreciation and Amortization									6.7	6.9	6.6
Operating Segments | Corporate and Other Segment
Segment Reporting Information
Premium Income									0.0	0.0	0.0
Net Investment Income									21.4	18.5	25.6
Other Income									2.9	4.9	2.9
Adjusted Operating Revenue									24.3	23.4	28.5
Adjusted Operating Income (Loss)									(146.8)	(163.0)	(136.7)
Interest and Debt Expense									153.2	159.1	146.2
Depreciation and Amortization									$ 0.9	$ 1.0	$ 0.9