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Debt - Additional Information Domain (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2007	Dec. 31, 1998	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2013
Debt Narrative Detail Information
Repayments of Principal in 2018							$ 200.0
Repayments of Principal in 2020							400.0
Repayments of Principal in 2021							350.0
Repayments of Principal Thereafter							2,071.1
Long-term Debt, Excluding Current Maturities		$ 2,449.4					2,999.4
Current Maturities of Long-term Debt		352.0					0.0
Proceeds from Issuance of Debt	$ 600.0
Payments of Debt Extinguishment Costs	$ 0.0	0.0	$ 13.2
Notional Amount of Derivatives		1,470.3	1,687.4				1,052.0		$ 1,477.4
Unum Group Percentage Ownership of Provident Financing Trust	100.00%
Interest Paid	$ 153.6	146.9	145.9
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
Debt Narrative Detail Information
Notional Amount of Derivatives		600.0					250.0
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)							27.6
Periodic Principal Payments	$ 64.0	$ 74.4	41.6
Long-term Debt
Debt Narrative Detail Information
Stated Interest Rate of Debt		6.85%
Extinguishment of Debt, Amount		$ 145.0
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
Secured Notes acquired 2016 \| Senior Notes
Debt Narrative Detail Information
Long-term Debt, Excluding Current Maturities		0.0					3.5
Notes Issued in 2009
Debt Narrative Detail Information
Current Maturities of Long-term Debt		350.0					0.0
Stated Interest Rate of Debt								7.125%
Notes Payable Due 2021
Debt Narrative Detail Information
Stated Interest Rate of Debt								3.00%
Proceeds from Issuance of Debt	350.0
Notes Payable due 2042
Debt Narrative Detail Information
Stated Interest Rate of Debt								5.75%
Proceeds from Issuance of Debt	$ 250.0
Notes Payable due 2042 \| Senior Notes
Debt Narrative Detail Information
Long-term Debt, Excluding Current Maturities		250.0					500.0
Stated Interest Rate of Debt								5.75%
Notes Issued in 2012
Debt Narrative Detail Information
Long-term Debt, Excluding Current Maturities							250.0
Stated Interest Rate of Debt								5.75%
Notes Issued in 2005
Debt Narrative Detail Information
Current Maturities of Long-term Debt		151.9
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
Long-term Debt, Excluding Current Maturities		$ 226.5					$ 226.5
Stated Interest Rate of Debt	7.405%							7.405%
Liquidation value per capital security						$ 1,000