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Debt - Additional Information Domain (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2007	Dec. 31, 1998	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2015
Debt Narrative Detail Information
Repayments of Principal in 2020						$ 400.0
Repayments of Principal in 2021						350.0
Repayments of Principal in 2023						2.0
Repayments of Principal Thereafter						2,245.6
Current Maturities of Long-term Debt		$ 199.9				0.0
Long-term Debt, Excluding Current Maturities		2,738.4				2,971.3
Notional Amount of Derivatives		904.5	$ 1,052.0			799.2			$ 1,470.3
Unum Group Percentage Ownership of Provident Financing Trust							100.00%
Interest Paid, Including Capitalized Interest, Operating and Investing Activities	$ 167.3	154.4	153.6
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
Secured Notes acquired 2016 \| Long-term Debt
Debt Narrative Detail Information
Extinguishment of Debt, Amount	3.4
Notes Issued in 2018 with a 2058 Maturity Date \| Junior Subordinated Debt
Debt Narrative Detail Information
Stated Interest Rate of Debt								6.25%
Proceeds from Issuance of Debt	$ 300.0
Long-term Debt, Excluding Current Maturities		0.0				300.0
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%
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)						73.8
Periodic Principal Payments	$ 60.0	60.0	$ 64.0
Receive Fixed/Pay Variable \| Interest Rate Swaps
Debt Narrative Detail Information
Notional Amount of Derivatives		250.0				$ 250.0
Receive Variable/Pay Fixed \| Interest Rate Swaps
Debt Narrative Detail Information
Loss on Contract Termination		$ 0.1