Fair Value Measurements -Narratives (Details) (USD $)	Jun. 30, 2014	Dec. 31, 2013
Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt fair value	$ 1,148,700,000	$ 1,161,900,000
Reported Value Measurement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt fair value	1,124,000,000	1,154,000,000
Alabama Gas Corporation | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt fair value	266,600,000	258,800,000
Alabama Gas Corporation | Reported Value Measurement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt fair value	249,800,000	249,900,000
Senior Loans | Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate	2.6675%
Senior Loans | Swap | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate cash flow hedge liability at fair value	1,400,000	1,800,000
Senior Loans | Senior Term Loans, Due March 31, 2014 to November 29, 2016 | Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Face amount	$ 190,000,000