Long-Term Debt and Notes Payable (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Debt Instrument [Line Items]
Long-term debt	$ 1,154,028	$ 1,155,246
Less amounts due within one year	50,000	1,000
Less unamortized debt discount	500	546
Long-term debt	1,103,528	1,153,700
Medium-term Notes | Medium-term Notes, Series A and B, interest ranging from 7.125% to 7.6%, for notes due July 24, 2017 to February 15, 2028
Debt Instrument [Line Items]
Long-term debt	154,000	155,000
Debt instrument, interest rate, minimum	7.125%
Debt instrument, interest rate, maximum	7.60%
Medium-term Notes | Medium-term Notes, Series A, interest of 7.57%
Debt Instrument [Line Items]
Debt instrument, interest rate		7.57%
Notes | 5% Notes, due October 1, 2013
Debt Instrument [Line Items]
Long-term debt	50,000	50,000
Debt instrument, interest rate	5.00%
Senior Notes | 4.625% Notes, due September 1, 2021
Debt Instrument [Line Items]
Long-term debt	400,000	400,000
Debt instrument, interest rate	4.625%
Senior Term Loans | Senior Term Loans, (floating rate interest LIBOR plus 1.375%; 1.59% at December 31, 2012), due March 31, 2014 to November 29, 2016
Debt Instrument [Line Items]
Long-term debt	300,000	300,000
Debt Instrument, basis spread on variable rate	1.375%
Floating rate interest, plus basis points	1.59%
Alabama Gas Corporation
Debt Instrument [Line Items]
Less amounts due within one year	0	0
Long-term debt	250,028	250,246
Alabama Gas Corporation | Notes | 5.20% Notes, due January 15, 2020
Debt Instrument [Line Items]
Long-term debt	40,000	40,000
Debt instrument, interest rate	5.20%
Alabama Gas Corporation | Notes | 5.70% Notes, due January 15, 2035
Debt Instrument [Line Items]
Long-term debt	35,028	35,246
Debt instrument, interest rate	5.70%
Alabama Gas Corporation | Notes | 5.368% Notes, due December 1, 2015
Debt Instrument [Line Items]
Long-term debt	80,000	80,000
Debt instrument, interest rate	5.368%
Alabama Gas Corporation | Notes | 5.90% Notes, due January 15, 2037
Debt Instrument [Line Items]
Long-term debt	45,000	45,000
Debt instrument, interest rate	5.90%
Alabama Gas Corporation | Notes | 3.86% Notes, due December 21, 2021
Debt Instrument [Line Items]
Long-term debt	$ 50,000	$ 50,000
Debt instrument, interest rate	3.86%