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Debt - Additional Information Domain (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2007	Dec. 31, 2006	Dec. 31, 1998	Dec. 31, 2011
Debt Narrative Detail Information
Repayments of Principal in Next Twelve Months	$ 151.9
Repayments of Principal in 2016	350.0
Repayments of Principal in 2018	200.0
Repayments of Principal Thereafter	2,081.0
Repayments of Long-term Debt	186.6us-gaap_RepaymentsOfLongTermDebt	116.2us-gaap_RepaymentsOfLongTermDebt	70.0us-gaap_RepaymentsOfLongTermDebt
Payments of Debt Extinguishment Costs	13.2	0	0
Notional Amount of Derivatives	1,687.4invest_DerivativeNotionalAmount	1,477.4invest_DerivativeNotionalAmount	1,432.8invest_DerivativeNotionalAmount				1,413.0invest_DerivativeNotionalAmount
Unum Group Percentage Ownership of Provident Financing Trust	100.00%
Interest Paid	145.9us-gaap_InterestPaid	144.6us-gaap_InterestPaid	139.6us-gaap_InterestPaid
Line of Credit Facility, Maximum Borrowing Capacity	400.0
Line Of Credit Facility Maximum Borrowing Capacity If Requested	600.0
Letters of Credit Outstanding	2.1	0
Amount Outstanding on Line of Credit	0us-gaap_LineOfCredit	0us-gaap_LineOfCredit
Receive Fixed/Pay Variable \| Swaps
Debt Narrative Detail Information
Notional Amount of Derivatives	600.0	600.0
Northwind Holdings LLC
Debt Narrative Detail Information
Proceeds from Issuance of Senior Long-term Debt				800.0
Basis Spread on LIBOR	0.78%
Debt service coverage account (DSCA)	5.4
Periodic Principal Payments	41.6	60.0
Tailwind Holdings LLC
Debt Narrative Detail Information
Proceeds from Issuance of Senior Long-term Debt					130.0
Periodic Principal Payments			10.0
Repayments of Long-term Debt		62.5
Gains (Losses) on Extinguishment of Debt		4.0
Long-term Debt
Debt Narrative Detail Information
Extinguishment of Debt, Amount	145.0
Debt Instrument, Interest Rate, Stated Percentage	6.85%
Payments of Debt Extinguishment Costs	13.2
Repayment of Long-term Debt, Including Cost of Make-whole Call Premium	158.2
Hedge Gain Reclassification from AOCI to Earnings	13.1
Notes Issued in 1998 with 2038 Maturity Date \| Junior Subordinated Debt
Debt Narrative Detail Information
Proceeds from Issuance of Senior Long-term Debt						$ 300.0
Debt Instrument, Interest Rate, Stated Percentage	7.405%
Liquidation value per capital security	$ 1,000