Long-Term Debt and Notes Payable (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Long-term debt	$ 1,403,923	$ 1,154,028
Less amounts due within one year	60,000	50,000
Less unamortized debt discount	459	500
Long-term debt	1,343,464	1,103,528
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	154,000
Debt instrument, interest rate, minimum	7.125%
Debt instrument, interest rate, maximum	7.60%
Notes \| 5% Notes
Debt Instrument [Line Items]
Long-term debt	0	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 1.625%; 1.792% at December 31, 2013), due March 31, 2014 to December 17, 2017
Debt Instrument [Line Items]
Long-term debt	600,000	0
Floating rate interest, plus basis points	1.792%
Senior Term Loans \| Senior Term Loans, (floating rate interest LIBOR plus 1.625%; 1.792% at December 31, 2013), due March 31, 2014 to December 17, 2017 \| LIBOR
Debt Instrument [Line Items]
Debt Instrument, basis spread on variable rate	1.625%
Senior Term Loans \| Senior Term Loans, (floating rate interest LIBOR plus 1.375%)
Debt Instrument [Line Items]
Long-term debt	0	300,000
Senior Term Loans \| Senior Term Loans, (floating rate interest LIBOR plus 1.375%) \| LIBOR
Debt Instrument [Line Items]
Debt Instrument, basis spread on variable rate	1.375%
Alabama Gas Corporation
Debt Instrument [Line Items]
Long-term debt	249,923	250,028
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	34,923	35,028
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%

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

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- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

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Maximum contractual interest rate for funds borrowed, under the debt agreement.

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Minimum contractual interest rate for funds borrowed, under the debt agreement.

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The amount of debt discount that was originally recognized at the issuance of the instrument that has yet to be amortized.

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Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Amount of long-term debt, after unamortized discount or premium, scheduled to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Carrying amount of long-term debt, net of unamortized discount or premium, excluding amounts to be repaid within one year or the normal operating cycle, if longer (current maturities). Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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The interest rate applicable to the portion of the carrying amount of long-term borrowings outstanding as of the balance sheet date, including current maturities, which accrues interest at a rate subject to change from time to time.

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