Commitments and Contingencies (Commitments and Agreements) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Long-term Purchase Commitment [Line Items]
Number of agreements to secure drilling rigs	3
Agreement Termination
Long-term Purchase Commitment [Line Items]
Possible resulting exposure form discontinue use of drilling rigs	$ 21.9
Crude Oil
Long-term Purchase Commitment [Line Items]
Oil and Gas Delivery Commitments and Contracts, Remaining Contractual Volume	33.4
Alabama Gas Corporation | Natural Gas, Delivery and Storage
Long-term Purchase Commitment [Line Items]
Long-term contracts, amount	59
Long-term contract, period end	September 2024
Long-term contracts expense recognized in financial statements	$ 51	$ 51	$ 52
Alabama Gas Corporation | Natural Gas
Long-term Purchase Commitment [Line Items]
Long-term contract, period end	August 2020
Long-term contracts, minimum quantity commitments	171