Deferred Acquisition Costs by Segment (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Beginning of Year	$ 1,677.1	$ 1,816.1	$ 1,784.3
Capitalized	467.3	442.5	422.5
Amortization	(378.7)	(365.7)	(373.3)
Impairment of Deferred Acquisition Costs	0	(196.0)	0
Adjustment Related to Unrealized Investment Gains/Losses	(12.0)	(19.6)	(16.1)
Deferred Policy Acquisition Costs Foreign Currency Translation Adjustment And Other	1.8	(0.2)	(1.3)
End of Year	1,755.5	1,677.1	1,816.1
Unum U.S.
Beginning of Year	971.8	943.7	932.4
Capitalized	249.2	220.3	207.8
Amortization	(196.5)	(188.1)	(192.6)
Impairment of Deferred Acquisition Costs		0
Adjustment Related to Unrealized Investment Gains/Losses	(0.2)	(4.1)	(3.9)
Deferred Policy Acquisition Costs Foreign Currency Translation Adjustment And Other	0	0	0
End of Year	1,024.3	971.8	943.7
Unum U.K.
Beginning of Year	40.9	41.0	40.8
Capitalized	11.8	15.4	15.1
Amortization	(15.7)	(15.3)	(13.6)
Impairment of Deferred Acquisition Costs		0
Adjustment Related to Unrealized Investment Gains/Losses	0	0	0
Deferred Policy Acquisition Costs Foreign Currency Translation Adjustment And Other	1.8	(0.2)	(1.3)
End of Year	38.8	40.9	41.0
Colonial Life
Beginning of Year	664.4	628.0	593.5
Capitalized	206.3	203.1	196.8
Amortization	(166.5)	(151.2)	(150.1)
Impairment of Deferred Acquisition Costs		0
Adjustment Related to Unrealized Investment Gains/Losses	(11.8)	(15.5)	(12.2)
Deferred Policy Acquisition Costs Foreign Currency Translation Adjustment And Other	0	0	0
End of Year	692.4	664.4	628.0
Closed Block
Beginning of Year	0	203.4	217.6
Capitalized	0	3.7	2.8
Amortization	0	(11.1)	(17.0)
Impairment of Deferred Acquisition Costs		(196.0)
Adjustment Related to Unrealized Investment Gains/Losses	0	0	0
Deferred Policy Acquisition Costs Foreign Currency Translation Adjustment And Other	0	0	0
End of Year	$ 0	$ 0	$ 203.4