Regulatory Assets and Liabilities, Regulatory Assets (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 464	$ 424
Regulatory Asset, Noncurrent		3,326	3,005
Past expenditures not currently earning a return		178	212
Pension and Retiree Medical Obligations
Regulatory Assets [Line Items]
Regulatory Asset, Current		87	91
Regulatory Asset, Noncurrent		$ 1,500	1,499
Regulatory asset, remaining amortization period		Various
Net AROs
Regulatory Assets [Line Items]
Regulatory Asset, Current	[1]	$ 0	0
Regulatory Asset, Noncurrent	[1]	$ 452	301
Regulatory asset, remaining amortization period		Plant lives
Excess deferred taxes - TCJA
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 0	0
Regulatory Asset, Noncurrent		$ 296	254
Regulatory asset, remaining amortization period		Various
Recoverable Deferred Taxes on AFUDC Recorded in Plant
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 0	0
Regulatory Asset, Noncurrent		$ 264	244
Regulatory asset, remaining amortization period		Plant lives
Environmental Remediation Costs
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 17	16
Regulatory Asset, Noncurrent		$ 155	165
Regulatory asset, remaining amortization period		Various
Depreciation Differences
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 18	20
Regulatory Asset, Noncurrent		$ 107	69
Regulatory Asset Amortization Period, minimum		1 year
Regulatory Asset Amortization Period, maximum		13 years
Benson purchase power agreement termination and asset purchase
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 10	0
Regulatory Asset, Noncurrent		$ 86	0
Regulatory asset, remaining amortization period		10 years
Contract Valuation Adjustments
Regulatory Assets [Line Items]
Regulatory Asset, Current	[2]	$ 17	21
Regulatory Asset, Noncurrent	[2]	$ 77	93
Regulatory asset, remaining amortization period		Term of related contract
Laurentian Energy Authority purchase power agreement termination
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 18	0
Regulatory Asset, Noncurrent		$ 73	0
Regulatory asset, remaining amortization period		5 years
Purchased Power Agreements
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 4	3
Regulatory Asset, Noncurrent		$ 63	67
Regulatory asset, remaining amortization period		Term of related contract
Prairie Island Nuclear Plant Extended Power Uprate
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 3	3
Regulatory Asset, Noncurrent		$ 56	58
Regulatory asset, remaining amortization period		16 years
Losses on Reacquired Debt
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 4	5
Regulatory Asset, Noncurrent		$ 44	48
Regulatory asset, remaining amortization period		Term of related contract
State Commission Adjustments
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 1	1
Regulatory Asset, Noncurrent		$ 29	29
Regulatory asset, remaining amortization period		Plant lives
Conservation Programs
Regulatory Assets [Line Items]
Regulatory Asset, Current	[3]	$ 42	50
Regulatory Asset, Noncurrent	[3]	$ 28	32
Regulatory Asset Amortization Period, minimum		1 year
Regulatory Asset Amortization Period, maximum		2 years
Property Tax
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 14	8
Regulatory Asset, Noncurrent		$ 10	24
Regulatory asset, remaining amortization period		Various
Nuclear Refueling Outage Costs
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 37	49
Regulatory Asset, Noncurrent		$ 14	20
Regulatory Asset Amortization Period, minimum		1 year
Regulatory Asset Amortization Period, maximum		2 years
Deferred Purchased Natural Gas and Electric Energy Costs
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 57	21
Regulatory Asset, Noncurrent		$ 13	13
Regulatory Asset Amortization Period, minimum		1 year
Regulatory Asset Amortization Period, maximum		3 years
Renewable Resources and Environmental Initiatives
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 39	48
Regulatory Asset, Noncurrent		$ 9	10
Regulatory Asset Amortization Period, minimum		1 year
Regulatory Asset Amortization Period, maximum		2 years
Sales True Up and Revenue Decoupling
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 38	37
Regulatory Asset, Noncurrent		$ 7	12
Regulatory Asset Amortization Period, minimum		1 year
Regulatory Asset Amortization Period, maximum		2 years
Gas Pipeline Inspection and Remediation Costs
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 28	24
Regulatory Asset, Noncurrent		$ 3	12
Regulatory Asset Amortization Period, minimum		1 year
Regulatory Asset Amortization Period, maximum		2 years
Other Regulatory Assets
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 30	27
Regulatory Asset, Noncurrent		$ 40	$ 55
Regulatory asset, remaining amortization period		Various

[1] Includes amounts recorded for future recovery of AROs, less amounts recovered through nuclear decommissioning accruals and gains from decommissioning investments.

[2] Includes the fair value of certain long-term PPAs used to meet energy capacity requirements and valuation adjustments on natural gas commodity purchases.

[3] Includes costs for conservation programs, as well as incentives allowed in certain jurisdictions.