Reconciliation of Operating Revenue and Operating Income by Segment to Total Revenue and Income Before Income Tax (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting, Revenue Reconciling Item
Net Realized Investment Gain (Loss)	$ (2.7)	$ (26.6)	$ 0.8	$ (15.3)	$ (17.3)	$ 1.2	$ 25.9	$ 6.3	$ (43.8)	$ 16.1	$ 6.8
Total Revenue	2,722.4	2,657.8	2,703.7	2,647.4	2,639.0	2,614.3	2,656.8	2,614.4	10,731.3	10,524.5	10,368.6
Net Realized Investment Gain (Loss)	(2.7)	(26.6)	0.8	(15.3)	(17.3)	1.2	25.9	6.3	(43.8)	16.1	6.8
Reserve Increase for Long-term Care					(698.2)
Unclaimed Death Benefit Reserve Increase (Decrease) Before Taxes											(95.5)
Income Before Income Tax	$ 316.6	$ 298.9	$ 319.8	$ 303.0	$ (454.6)	$ 317.0	$ 349.8	$ 329.8	1,238.3	542.0	1,220.0
Operating Segments
Segment Reporting, Revenue Reconciling Item
Operating Revenue									10,775.1	10,508.4	10,361.8
Net Realized Investment Gain (Loss)									(43.8)	16.1	6.8
Total Revenue									10,731.3	10,524.5	10,368.6
Operating Income									1,294.0	1,307.3	1,256.6
Net Realized Investment Gain (Loss)									(43.8)	16.1	6.8
Non-operating Retirement-related Loss									(11.9)	(70.0)	(32.9)
Make-whole Premium on Early Repayment of Debt									0.0	(13.2)	0.0
Reserve Increase for Long-term Care									0.0	(698.2)	0.0
Waiver of Premium Benefit Reserve Increase (Decrease) Before Tax									0.0	0.0	85.0
Income Before Income Tax									1,238.3	542.0	1,220.0
Unum U.S.
Segment Reporting, Revenue Reconciling Item
Unclaimed Death Benefit Reserve Increase (Decrease) Before Taxes											(75.4)
Unum U.S. | Operating Segments
Segment Reporting, Revenue Reconciling Item
Operating Revenue									5,944.5	5,660.7	5,564.8
Operating Income									850.0	844.9	848.8
Unclaimed Death Benefit Reserve Increase (Decrease) Before Taxes									0.0	0.0	(75.4)
Colonial Life | Operating Segments
Segment Reporting, Revenue Reconciling Item
Operating Revenue									1,484.1	1,419.3	1,376.5
Operating Income									309.1	299.0	283.6
Unclaimed Death Benefit Reserve Increase (Decrease) Before Taxes									$ 0.0	$ 0.0	$ (20.1)