Stockholders' Equity and Earnings Per Common Share - Additional Information (Detail) (USD $) In Millions, except Share data	3 Months Ended			9 Months Ended		12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended
	Sep. 30, 2011	Mar. 31, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010	Sep. 30, 2011 Share Repurchase Program Expire In August 2012	Dec. 31, 2010 Share Repurchase Program Expire in May 2011 [Member]	Sep. 30, 2011 Accelerated share repurchase agreement	Mar. 18, 2011 Accelerated share repurchase agreement
Stockholders Equity Note [Line Items]
Outstanding stock options, exercise prices range, lower limit	$ 11.37			$ 11.37
Outstanding stock options, exercise prices range, upper limit	$ 26.29			$ 26.29
Outstanding Nonvested stock awards, grant prices range, lower limit	$ 10.59			$ 10.59
Outstanding Nonvested stock awards, grant prices range, upper limit	$ 26.31			$ 26.31
Outstanding Performance restricted stock units, threshold stock price	$ 26.00			$ 26.00
Antidilutive common shares not included in the computation of dilutive earnings per share	2,400,000		3,600,000	2,100,000	3,600,000
Authorized repurchase of common stock, value							$ 1,000.0	$ 500.0
Treasury Stock Acquired, Shares			9,500,000		15,200,000	16,400,000	7,700,000		7,100,000
Treasury Stock Acquired in Open Market, Value	250.2			419.9
Common stock repurchased			197.7		327.5	356.0			200.0
Stock Repurchase Program Remaining Authorized Repurchase Amount				524.7
Shares repurchased cost, Commissions	$ 0.2		$ 0.2	$ 0.3	$ 0.3	$ 0.3
Price adjustment receipt in shares of Unum Group common stock										600,000
Treasury Stock Shares Retired		7,700,000
Treasury Stock Acquired in Open Market, Shares	11,100,000			17,700,000
Preferred stock, par value	$ 0.10			$ 0.10
Preferred stock, authorized	25,000,000			25,000,000