Long-Term Debt and Notes Payable (Textual) (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Debt Instrument [Line Items]
Cross default provision, minimum threshold amount	$ 10,000,000
Interest expense	44,822,000	39,222,000	39,379,000
Senior Term Loans | Swap
Debt Instrument [Line Items]
Fixed interest rate exchanged for variable interest rate under swap agreement	2.4175%
Senior Term Loans | Swap | Level 1
Debt Instrument [Line Items]
Interest rate swap liability, fair value	1,500,000
Senior Term Loans | Senior Term Loans, (floating rate interest LIBOR plus 1.375%; currently 1.67% at December 31, 2011), due March 31, 2014 to November 29, 2016
Debt Instrument [Line Items]
Long-term debt	300,000,000
Senior Term Loans | Senior Term Loans, (floating rate interest LIBOR plus 1.375%; currently 1.67% at December 31, 2011), due March 31, 2014 to November 29, 2016 | Swap
Debt Instrument [Line Items]
Long-term debt	200,000,000
Senior Notes | 4.625% Notes, due September 1, 2021
Debt Instrument [Line Items]
Long-term debt	400,000,000
Debt instrument, interest rate	4.625%
Alabama Gas Corporation
Debt Instrument [Line Items]
Cross default provision, minimum threshold amount	10,000,000
Interest expense	14,740,000	13,894,000	13,714,000
Alabama Gas Corporation | Notes | 3.86% Notes, due December 21, 2021
Debt Instrument [Line Items]
Long-term debt	$ 50,000,000
Debt instrument, interest rate	3.86%