Summary of Significant Accounting Policies - Purchase of the Plant Daniel Combined Cycle Generating Units (Details) (Mississippi Power [Member], Plant Daniel Units 3 and 4 [Member], USD $) In Thousands, unless otherwise specified	Oct. 20, 2011
Mississippi Power [Member] | Plant Daniel Units 3 and 4 [Member]
Significant Acquisitions and Disposals [Line Items]
Assumptions of debt obligations	$ 270,000
Fair value adjustment at date of purchase	76,051
Total debt	346,051
Cash payment for the purchase	84,803
Total value of Plant Daniel Units 3 and 4	$ 430,854