Supplemental Cash Flow Information (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Supplemental Cash Flow Information [Line Items]
Interest paid, net of amount capitalized	$ 65,143	$ 61,379	$ 33,601
Income taxes paid	25,081	17,170	9,432
Noncash investing activities:
Accrued development, exploration costs and other capital	99,128	120,024	72,030
Capitalized depreciation	66	80	93
Capitalized asset retirement obligations costs	3,574	4,409	4,927
Allowance for funds used during construction	698	623	807
Capital lease obligations	0	5,072	0
Issuance of common stock for employee benefit plans	1,015	838	822
Treasury stock acquired in connection with tax withholdings	977	277	713
Alabama Gas Corporation
Supplemental Cash Flow Information [Line Items]
Interest paid, net of amount capitalized	13,465	13,513	12,385
Income taxes paid	23,138	16,796	5,143
Interest expense (revenue) on affiliated company debt, net	(18)	295	376
Noncash investing activities:
Accrued development, exploration costs and other capital	5,505	3,536	2,229
Capitalized depreciation	66	80	93
Capitalized asset retirement obligations costs	802	415	300
Allowance for funds used during construction	$ 698	$ 623	$ 807