Reconciliation of Policy and Contract Benefits and Reserves for Future Policy and Contract Benefits (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Insurance [Abstract]
Policy and Contract Benefits	$ 1,529.3	$ 1,511.0	$ 1,484.6
Reserves for Future Policy and Contract Benefits	45,929.4	43,099.1	44,694.4
Total Policy and Contract Benefits and Reserves for Future Policy and Contract Benefits	47,458.7	44,610.1	46,179.0
Life Reserves for Future Policy and Contract Benefits	7,850.9	7,740.5	7,571.1
Accident and Health Active Life Reserves	9,263.5unm_AccidentAndHealthActiveLifeReserves	8,225.5unm_AccidentAndHealthActiveLifeReserves	7,763.3unm_AccidentAndHealthActiveLifeReserves
Unrealized Adjustment to Reserves for Future Policy and Contract Benefits	6,150.3	4,108.5	6,277.5
Liability for Claims and Claims Adjustment Expense	$ 24,194.0	$ 24,535.6	$ 24,567.1	$ 24,586.5

Accident and Health Active Life Reserves

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Life Reserves for Future Policy and Contract Benefits

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Unrealized Adjustment to Reserves for Future Policy and Contract Benefits

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No definition available.

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The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 40

-Section 50

-Paragraph 3

-Subparagraph (a),(b),(c)

-URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437

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The amount needed to reflect the estimated ultimate cost of settling claims relating to disability, accident and health insurance insured events that have occurred on or before a particular date (ordinarily, the balance sheet date) and the amount needed to provide for the estimated ultimate cost required to investigate and settle claims relating to insured events that have occurred on or before a particular date (ordinarily, the balance sheet date), whether or not reported to the insurer at that date.

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Amount of accrued obligation to policyholders that relates to insured events for long-duration contracts.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 40

-Section 25

-Paragraph 11

-URI http://asc.fasb.org/extlink&oid=28366191&loc=d3e13145-158430

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

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-Topic 944

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-Section S99

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The aggregate amount of policy reserves (provided for future obligations including unpaid claims and claims adjustment expenses) and policy benefits (liability for future policy benefits) as of the balance sheet date; grouped amount of all the liabilities associated with the company's insurance policies.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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-SubTopic 235

-Section S99

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574960-122915

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