Commitments and Contingencies (Lease Obligations) (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Operating Leased Assets [Line Items]
Total lease payments	$ 19,100,000	$ 18,600,000	$ 21,500,000
Operating Leases, Future Minimum Payments Due [Abstract]
2012	5,047,000
2013	4,820,000
2014	4,013,000
2015	3,664,000
2016	2,274,000
2017 and thereafter	12,729,000
Alabama Gas Corporation
Operating Leased Assets [Line Items]
Leasing term	25
Total lease payments	2,300,000	2,100,000	2,200,000
Lease expense paid by parent company	1,000,000
Operating Leases, Future Minimum Payments Due [Abstract]
2012	3,238,000
2013	3,184,000
2014	3,180,000
2015	3,201,000
2016	2,093,000
2017 and thereafter	12,729,000
Alabama Gas Corporation | Company's headquarters
Operating Leased Assets [Line Items]
Payments associated with leasing of Company's headquarters	$ 11,400,000