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Debt - Additional Information Domain (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2007	Dec. 31, 2006	Dec. 31, 1998	Dec. 31, 2015	Dec. 31, 2015	Dec. 31, 2015	Dec. 31, 2012
Debt Narrative Detail Information
Repayments of Principal in Next Twelve Months								$ 350.0
Repayments of Principal in 2018								200.0
Repayments of Principal in 2020								400.0
Repayments of Principal Thereafter								1,881.6
Repayments of Long-term Debt	$ 226.3	$ 186.6	$ 116.2
Payments of Debt Extinguishment Costs	$ 0.0	13.2	0.0
Current Maturities of Long-term Debt		151.9						352.4
Notional Amount of Derivatives		1,687.4	1,477.4					1,470.3		$ 1,432.8
Unum Group Percentage Ownership of Provident Financing Trust	100.00%
Interest Paid	$ 146.9	145.9	144.6
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						2.1
Amount Outstanding on Line of Credit		0.0						0.0
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	$ 74.4	41.6	60.0
Tailwind Holdings LLC
Debt Narrative Detail Information
Proceeds from Issuance of Senior Long-term Debt					$ 130.0
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
Stated Interest Rate of Debt		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	$ 27.7	13.1
Notes Issued in 2005
Debt Narrative Detail Information
Stated Interest Rate of Debt									6.85%
Current Maturities of Long-term Debt		$ 151.9						$ 0.0
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
Stated Interest Rate of Debt	7.405%								7.405%
Liquidation value per capital security							$ 1,000