Common Stock Plans (Other Plans) (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended			3 Months Ended		12 Months Ended		3 Months Ended	12 Months Ended		12 Months Ended
	Dec. 31, 2011 Petrotech Incentive Plan Stock Equivalent Units	Dec. 31, 2010 Petrotech Incentive Plan Stock Equivalent Units	Dec. 31, 2009 Petrotech Incentive Plan Stock Equivalent Units	Sep. 30, 2009 Petrotech Incentive Plan Stock Equivalent Units Three years vesting period	Mar. 31, 2009 Petrotech Incentive Plan Stock Equivalent Units Three years vesting period	Dec. 31, 2011 Petrotech Incentive Plan Stock Equivalent Units Three years vesting period	Dec. 31, 2010 Petrotech Incentive Plan Stock Equivalent Units Three years vesting period	Mar. 31, 2009 Petrotech Incentive Plan Stock Equivalent Units Two years vesting period	Dec. 31, 2011 Petrotech Incentive Plan Stock Equivalent Units Two years vesting period	Dec. 31, 2011 1997 Deferred Compensation Plan Management	Dec. 31, 2011 1992 Energen Corporation Directors Stock Plan Stock options Director	Dec. 31, 2010 1992 Energen Corporation Directors Stock Plan Stock options Director	Dec. 31, 2009 1992 Energen Corporation Directors Stock Plan Stock options Director
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity instruments awarded in period (in units)				938	2,911	6,314	2,442	900
Share-based compensation, vesting period						3 years			2 years
Share-based compensation expense	$ 0.2	$ 0.2	$ 1.0
Deferred Compensation Arrangements [Abstract]
Deferred compensation, reserved for issuance										706,355
Granted, Shares											12,420	15,400	12,000
Number of shares remaining for issuance											162,904