Schedule II (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Expense Related to Revaluation of Federal Benefit - TCJA		$ 14
Reduced Expense Related to Revaluation of Federal Benefits - TCJA		4
Allowance for Bad Debts
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Jan. 1		55		$ 52		$ 51
Charged to costs and expenses		42		42		39
Charged to other accounts	[1]	16		11		10
Deductions from reserves	[2]	(58)		(50)		(48)
Balance at Dec. 31		55		55		52
NOL and Tax Credit Valuation Allowances
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Jan. 1		79		77		58
Charged to costs and expenses		9		7		9
Charged to other accounts		0		0		22	[3]
Deductions from reserves		(21)	[4]	(5)	[4]	(12)	[5]
Balance at Dec. 31		$ 67		$ 79		$ 77

[1] Recovery of amounts previously written off.

[2] Deductions related primarily to bad debt write-offs.

[3] Accrual of valuation allowances for North Dakota ITC, net of federal income tax benefit, that is offset to a regulatory liability and includes $14 million expense related to the revaluation of federal benefit as a result of the TCJA.

[4] Primarily the reductions to valuation allowances due to additional NOLs and tax credits now forecasted to be used prior to expiration.

[5] Primarily the reductions to valuation allowances for North Dakota ITC carryforwards, net of federal benefit, primarily due to a consolidated adjustment to the regulatory liability accrual referenced above; the change includes $4 million of reduced expense related to the revaluation of federal benefit as a result of TCJA.