Income Taxes - Impact of Unrecognized Tax Benefits on Effective Tax Rate, If Recognized (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Impact on effective tax rate
Tax positions impacting the effective tax rate	$ 20	$ 10	$ 10
Tax positions not impacting the effective tax rate	464	423	160
Balance of unrecognized tax benefits	484	433	170	$ 7
Mississippi Power [Member]
Impact on effective tax rate
Tax positions impacting the effective tax rate	1	(2)	4
Tax positions not impacting the effective tax rate	464	423	161
Balance of unrecognized tax benefits	$ 465	$ 421	$ 165	$ 4