Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information
Premium Income	$ 2,352.6	$ 2,331.2	$ 2,343.1	$ 2,338.7	$ 2,263.3	$ 2,251.8	$ 2,221.0	$ 2,250.0	$ 9,365.6	$ 8,986.1	$ 8,597.1
Net Investment Income	$ 616.3	$ 599.4	$ 624.9	$ 594.7	$ 608.6	$ 619.2	$ 623.6	$ 602.3	2,435.3	2,453.7	2,451.7
Other Income									221.2	198.2	197.7
Interest and Debt Expense									177.4	167.3	159.9
Operating Segments
Segment Reporting Information
Premium Income									9,365.6	8,986.1	8,597.1
Net Investment Income									2,435.3	2,453.7	2,451.7
Other Income									221.2	198.2	197.7
Adjusted Operating Revenue									12,022.1	11,638.0	11,246.5
Adjusted Operating Income (Loss)									1,432.6	1,418.1	1,423.3
Interest and Debt Expense									177.4	167.3	159.9
Depreciation and Amortization									727.5	673.8	636.1
Operating Segments | Unum US
Segment Reporting Information
Premium Income									6,016.6	5,736.4	5,443.5
Net Investment Income									739.4	778.7	811.2
Other Income									142.8	118.5	113.2
Adjusted Operating Revenue									6,898.8	6,633.6	6,367.9
Adjusted Operating Income (Loss)									1,031.1	1,014.6	1,009.5
Interest and Debt Expense									0.0	0.0	0.0
Depreciation and Amortization									422.8	389.6	371.3
Operating Segments | Unum International
Segment Reporting Information
Premium Income									630.5	568.8	513.0
Net Investment Income									122.5	117.2	120.2
Other Income									0.6	0.4	0.7
Adjusted Operating Revenue									753.6	686.4	633.9
Adjusted Operating Income (Loss)									107.9	113.9	111.7
Interest and Debt Expense									0.0	0.0	0.0
Depreciation and Amortization									18.7	17.6	17.3
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income									1,685.0	1,603.8	1,511.4
Net Investment Income									148.0	151.2	144.9
Other Income									3.4	1.2	1.1
Adjusted Operating Revenue									1,836.4	1,756.2	1,657.4
Adjusted Operating Income (Loss)									344.5	335.2	325.0
Interest and Debt Expense									0.0	0.0	0.0
Depreciation and Amortization									276.6	257.3	239.9
Operating Segments | Closed Block
Segment Reporting Information
Premium Income									1,033.5	1,077.1	1,129.2
Net Investment Income									1,404.9	1,377.1	1,354.0
Other Income									71.3	75.4	79.8
Adjusted Operating Revenue									2,509.7	2,529.6	2,563.0
Adjusted Operating Income (Loss)									137.7	125.5	123.9
Interest and Debt Expense									5.3	6.9	6.7
Depreciation and Amortization									7.7	8.3	6.7
Operating Segments | Corporate and Other Segment
Segment Reporting Information
Premium Income									0.0	0.0	0.0
Net Investment Income									20.5	29.5	21.4
Other Income									3.1	2.7	2.9
Adjusted Operating Revenue									23.6	32.2	24.3
Adjusted Operating Income (Loss)									(188.6)	(171.1)	(146.8)
Interest and Debt Expense									172.1	160.4	153.2
Depreciation and Amortization									$ 1.7	$ 1.0	$ 0.9