Reconciliation of Total Operating Revenue and Operating Income by Segment to Revenue and Net Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenue									$ 10,347.0	$ 10,459.2	$ 10,282.9
Realized Investment Gains (Losses)									6.8	56.2	(4.9)
Revenues	2,586.2	2,540.9	2,601.9	2,624.8	2,658.2	2,628.0	2,617.9	2,611.3	10,353.8	10,515.4	10,278.0
Operating Income (Loss)									1,241.8	1,239.7	1,323.2
Realized Investment Gains (Losses)									6.8	56.2	(4.9)
Non-operating Retirement-related Loss									(32.9)	(46.4)	(31.9)
Unclaimed Death Benefit Reserve Increase (Decrease) Before Taxes									(95.5)
Waiver of Premium Benefit Reserve Increase (Decrease) Before Tax									85.0
Impairment of Deferred Acquisition Costs									0	0	(196.0)
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	305.8	284.1	311.5	303.8	322.7	320.4	296.4	310.0	1,205.2	1,249.5	333.3
Unum U.S.
Segment Reporting, Revenue Reconciling Item [Line Items]
Unclaimed Death Benefit Reserve Increase (Decrease) Before Taxes									(75.4)	0	0
Colonial Life
Segment Reporting, Revenue Reconciling Item [Line Items]
Unclaimed Death Benefit Reserve Increase (Decrease) Before Taxes									(20.1)	0	0
Long-term Care
Segment Reporting, Revenue Reconciling Item [Line Items]
Impairment of Deferred Acquisition Costs									0	0	(769.6)
Individual Disability - Closed Block
Segment Reporting, Revenue Reconciling Item [Line Items]
Reserve Increase Due to Unlocking									0	0	(183.5)
Group Life | Unum U.S.
Segment Reporting, Revenue Reconciling Item [Line Items]
Unclaimed Death Benefit Reserve Increase (Decrease) Before Taxes									(49.1)
Waiver of Premium Benefit Reserve Increase (Decrease) Before Tax									$ 85.0	$ 0	$ 0