Summary of Significant Accounting Policies (Cash Equivalents, Short-term Investments, and Stock-Based Compensation) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cash equivalent, maturity period	3 months
Reclassification of Outstanding Checks, Effect on Cash and Cash Equivalents		$ 8.0
Reclassification of Outstanding Checks, Effect on Accounts Payable		7.3
Reclassification of Outstanding Checks, Effect on Accounts Receivable		0.7
Decrease in operating cash flows	(8.0)
Excess tax benefit from share-based compensation	0.6	1.0
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Short-term investments, maturity period	3 months
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Short-term investments, maturity period	1 year
Alabama Gas Corporation
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Reclassification of Outstanding Checks, Effect on Cash and Cash Equivalents		6.8
Reclassification of Outstanding Checks, Effect on Accounts Payable		6.1
Reclassification of Outstanding Checks, Effect on Accounts Receivable		0.7
Decrease in operating cash flows	$ 6.8