Deferred Acquisition Costs by Segment (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Beginning of Year	$ 2,521.1	$ 2,482.5	$ 2,472.4
Capitalized	628.3	607.7	593.6
Amortization	(533.8)	(547.1)	(526.2)
Impairment of Deferred Acquisition Costs	(289.8)	0	0
Adjustment Related to Unrealized Investment Gains/Losses	(24.7)	(20.0)	(62.9)
Deferred Policy Acquisition Costs Foreign Currency Translation Adjustment And Other	(0.2)	(2.0)	5.6
End of Year	2,300.9	2,521.1	2,482.5
Unum U.S.
Beginning of Year	1,362.2	1,351.5	1,341.4
Capitalized	333.8	323.2	321.6
Amortization	(298.7)	(307.9)	(293.8)
Impairment of Deferred Acquisition Costs	0
Adjustment Related to Unrealized Investment Gains/Losses	(5.0)	(4.6)	(17.7)
Deferred Policy Acquisition Costs Foreign Currency Translation Adjustment And Other	0	0	0
End of Year	1,392.3	1,362.2	1,351.5
Unum U.K.
Beginning of Year	58.2	58.9	54.7
Capitalized	30.6	28.3	29.1
Amortization	(29.2)	(27.0)	(30.5)
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	(0.2)	(2.0)	5.6
End of Year	59.4	58.2	58.9
Colonial Life
Beginning of Year	805.0	761.2	755.9
Capitalized	252.9	246.4	229.0
Amortization	(189.0)	(187.2)	(178.5)
Impairment of Deferred Acquisition Costs	0
Adjustment Related to Unrealized Investment Gains/Losses	(19.7)	(15.4)	(45.2)
Deferred Policy Acquisition Costs Foreign Currency Translation Adjustment And Other	0	0	0
End of Year	849.2	805.0	761.2
Closed Block
Beginning of Year	295.7	310.9	320.4
Capitalized	11.0	9.8	13.9
Amortization	(16.9)	(25.0)	(23.4)
Impairment of Deferred Acquisition Costs	(289.8)
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	$ 295.7	$ 310.9