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Debt - Additional Information Domain (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2007	Dec. 31, 1998	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2014
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							2,007.6
Current Maturities of Long-term Debt		$ 0.0					199.9
Proceeds from Issuance of Debt	$ 600.0
Long-term Debt, Excluding Current Maturities		2,999.4					2,738.4
Notional Amount of Derivatives		1,052.0	$ 1,470.3				904.5		$ 1,687.4
Unum Group Percentage Ownership of Provident Financing Trust	100.00%
Interest Paid	$ 154.4	153.6	146.9
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							250.0
Long-term Debt
Debt Narrative Detail Information
Stated Interest Rate of Debt		6.85%
Net Realized Investments Gain Loss \| Receive Variable/Pay Fixed
Debt Narrative Detail Information
Loss on Contract Termination	0.1
Secured Notes acquired 2016 \| Long-term Debt
Debt Narrative Detail Information
Extinguishment of Debt, Amount	3.4
Notes Issued in 2009
Debt Narrative Detail Information
Current Maturities of Long-term Debt		$ 350.0
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 Issued in 2012
Debt Narrative Detail Information
Stated Interest Rate of Debt								5.75%
Long-term Debt, Excluding Current Maturities							250.0
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
Stated Interest Rate of Debt	7.405%							7.405%
Long-term Debt, Excluding Current Maturities		226.5					226.5
Liquidation value per capital security						$ 1,000
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)							$ 40.9
Periodic Principal Payments	$ 60.0	$ 64.0	$ 74.4