Segment Information Deferred Acquisition Costs by Segment (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	$ 1,829.2
Deferred Policy Acquisition Cost, Amortization Expense	(440.8)	(418.9)	(378.7)
Deferred Acquisition Costs at End of Year	1,901.3	1,829.2
Operating Segments
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	1,829.2	1,755.5	1,677.1
Deferral of Acquisition Costs	524.0	466.8	467.3
Deferred Policy Acquisition Cost, Amortization Expense	(440.8)	(418.9)	(378.7)
Adjustment Related to Unrealized Investment (Gain) Loss	(9.2)	25.4	(12.0)
Deferred Policy Acquisition Costs Foreign Currency Translation Adjustment And Other	(1.9)	0.4	1.8
Deferred Acquisition Costs at End of Year	1,901.3	1,829.2	1,755.5
Operating Segments | Unum U.S.
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	1,051.5	1,024.3	971.8
Deferral of Acquisition Costs	292.7	252.0	249.2
Deferred Policy Acquisition Cost, Amortization Expense	(248.1)	(230.0)	(196.5)
Adjustment Related to Unrealized Investment (Gain) Loss	0.4	5.2	(0.2)
Deferred Policy Acquisition Costs Foreign Currency Translation Adjustment And Other	0	0	0
Deferred Acquisition Costs at End of Year	1,096.5	1,051.5	1,024.3
Operating Segments | Unum U.K.
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	34.3	38.8	40.9
Deferral of Acquisition Costs	10.5	9.8	11.8
Deferred Policy Acquisition Cost, Amortization Expense	(12.5)	(14.7)	(15.7)
Adjustment Related to Unrealized Investment (Gain) Loss	0	0	0
Deferred Policy Acquisition Costs Foreign Currency Translation Adjustment And Other	(1.9)	0.4	1.8
Deferred Acquisition Costs at End of Year	30.4	34.3	38.8
Operating Segments | Colonial Life
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	743.4	692.4	664.4
Deferral of Acquisition Costs	220.8	205.0	206.3
Deferred Policy Acquisition Cost, Amortization Expense	(180.2)	(174.2)	(166.5)
Adjustment Related to Unrealized Investment (Gain) Loss	(9.6)	20.2	(11.8)
Deferred Policy Acquisition Costs Foreign Currency Translation Adjustment And Other	0	0	0
Deferred Acquisition Costs at End of Year	$ 774.4	$ 743.4	$ 692.4