Income Taxes Income Tax Rate Change (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Jun. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Income Tax Rate Change [Line Items]
Federal statutory income tax rate	21.00%	35.00%		21.00%	35.00%	35.00%
Reclassification of long-term income tax receivable				$ (609)
Long-term income tax receivable [Member]
Income Tax Rate Change [Line Items]
Reclassification of long-term income tax receivable				(609)
Long-term income tax receivable adjustments				115
Iowa Senate File 2417, Rate Effective Starting 2021 [Member]
Income Tax Rate Change [Line Items]
Deferred Income Tax Expense (Benefit)				2
Deferred Tax Liabilities, Net	$ 61			61
Iowa Senate File 2417, Rate Effective Starting 2021 [Member] | Deferred Income Tax Charge [Member]
Income Tax Rate Change [Line Items]
Increase in Regulatory Liability				59
Iowa Senate File 2417, Rate Effective Starting 2021 [Member] | Long-term income tax receivable [Member]
Income Tax Rate Change [Line Items]
Long-term income tax receivable adjustments				$ 115
Tax Cuts and Jobs Act of 2017 [Member]
Income Tax Rate Change [Line Items]
Federal statutory income tax rate				21.00%
Increase (decrease) in provisional tax liabilities			$ 45
Deferred Income Tax Expense (Benefit)	37
Current Income Tax Expense (Benefit)	(37)
Increase (Decrease) in Deferred Income Taxes	14
Tax Cuts and Jobs Act of 2017 [Member] | Deferred Income Tax Charge [Member]
Income Tax Rate Change [Line Items]
Increase in Regulatory Liability	$ 14
IOWA | Iowa Senate File 2417, Rate Effective Starting 2021 [Member] | Maximum [Member]
Income Tax Rate Change [Line Items]
Statutory Income Tax Rate, State and Local, Percent				9.80%