Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information
Premium Income	$ 2,263.3	$ 2,251.8	$ 2,221.0	$ 2,250.0	$ 2,158.4	$ 2,153.6	$ 2,142.2	$ 2,142.9	$ 8,986.1	$ 8,597.1	$ 8,357.7
Net Investment Income	$ 608.6	$ 619.2	$ 623.6	$ 602.3	$ 619.8	$ 609.0	$ 620.5	$ 602.4	2,453.7	2,451.7	2,459.0
Other Income									198.2	197.7	205.6
Interest and Debt Expense									167.3	159.9	166.0
Operating Segments
Segment Reporting Information
Premium Income									8,986.1	8,597.1	8,357.7
Net Investment Income									2,453.7	2,451.7	2,459.0
Other Income									198.2	197.7	205.6
Adjusted Operating Revenue									11,638.0	11,246.5	11,022.3
Adjusted Operating Income (Loss)									1,418.1	1,423.3	1,323.5
Interest and Debt Expense									167.3	159.9	166.0
Depreciation and Amortization									673.8	636.1	600.2
Operating Segments | Unum US
Segment Reporting Information
Premium Income									5,736.4	5,443.5	5,240.9
Net Investment Income									778.7	811.2	828.7
Other Income									118.5	113.2	113.3
Adjusted Operating Revenue									6,633.6	6,367.9	6,182.9
Adjusted Operating Income (Loss)									1,014.6	1,009.5	914.2
Interest and Debt Expense									0.0	0.0	0.0
Depreciation and Amortization									389.6	371.3	353.0
Operating Segments | Unum International
Segment Reporting Information
Premium Income									568.8	513.0	529.3
Net Investment Income									117.2	120.2	118.1
Other Income									0.4	0.7	0.2
Adjusted Operating Revenue									686.4	633.9	647.6
Adjusted Operating Income (Loss)									113.9	111.7	128.6
Interest and Debt Expense									0.0	0.0	0.0
Depreciation and Amortization									17.6	17.3	16.4
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income									1,603.8	1,511.4	1,417.1
Net Investment Income									151.2	144.9	141.5
Other Income									1.2	1.1	1.2
Adjusted Operating Revenue									1,756.2	1,657.4	1,559.8
Adjusted Operating Income (Loss)									335.2	325.0	314.2
Interest and Debt Expense									0.0	0.0	0.0
Depreciation and Amortization									257.3	239.9	222.9
Operating Segments | Closed Block
Segment Reporting Information
Premium Income									1,077.1	1,129.2	1,170.4
Net Investment Income									1,377.1	1,354.0	1,352.2
Other Income									75.4	79.8	86.0
Adjusted Operating Revenue									2,529.6	2,563.0	2,608.6
Adjusted Operating Income (Loss)									125.5	123.9	129.5
Interest and Debt Expense									6.9	6.7	6.9
Depreciation and Amortization									8.3	6.7	6.9
Operating Segments | Corporate and Other Segment
Segment Reporting Information
Premium Income									0.0	0.0	0.0
Net Investment Income									29.5	21.4	18.5
Other Income									2.7	2.9	4.9
Adjusted Operating Revenue									32.2	24.3	23.4
Adjusted Operating Income (Loss)									(171.1)	(146.8)	(163.0)
Interest and Debt Expense									160.4	153.2	159.1
Depreciation and Amortization									$ 1.0	$ 0.9	$ 1.0