Supplemental Cash Flow Information (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Supplemental Cash Flow Information [Line Items]
Interest paid, net of amount capitalized	$ 61,379	$ 33,601	$ 37,517
Income taxes paid	17,170	9,432	83,894
Noncash investing activities:
Accrued development, exploration costs and other capital	120,024	72,030	75,167
Capitalized depreciation	80	93	116
Capitalized asset retirement obligations costs	4,409	4,927	4,194
Allowance for funds used during construction	623	807	808
Capital lease obligations	5,072	0	0
Issuance of common stock for employee benefit plans	838	822	5,765
Treasury stock acquired in connection with tax withholdings	277	713	2,894
Accretion expense	7,534	6,837	6,178
Alabama Gas Corporation
Supplemental Cash Flow Information [Line Items]
Interest paid, net of amount capitalized	13,513	12,385	11,653
Income taxes paid	16,796	5,143	13,063
Interest on affiliated company debt, net	295	376	274
Noncash investing activities:
Accrued development, exploration costs and other capital	3,536	2,229	2,592
Capitalized depreciation	80	93	116
Capitalized asset retirement obligations costs	415	300	161
Allowance for funds used during construction	$ 623	$ 807	$ 808