Regulatory Matters - Regulatory Liabilities (Details) - USD ($) $ in Millions		1 Months Ended			12 Months Ended
		Jul. 31, 2016	Oct. 31, 2015	Nov. 30, 2014	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Regulatory Liabilities [Line Items]
Federal statutory income tax rate					35.00%	35.00%	35.00%
Regulatory liabilities					$ 202	$ 187
Regulatory liabilities, noncurrent					7,309	2,933
Regulatory Liabilities					7,511	3,120
Deferred Income Tax Charge [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	[1]				$ 4,143	25
Weighted average remaining life	[1]				Various
Removal Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	[2]				$ 2,349	2,242
Weighted average remaining life	[2]				26.845466155811
Deferred net power costs [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities					$ 8	64
Weighted average remaining life					2
Asset Retirement Obligation Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities					$ 177	122
Weighted average remaining life					34.638418079096
Levelized depreciation [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities					$ 332	244
Weighted average remaining life					22
Impact fees [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities					$ 89	90
Weighted average remaining life					6
Employee benefit plans [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	[3]				$ 69	25
Weighted average remaining life	[3]				10.6086956521739
Unrealized gain on regulated derivative contracts [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities					$ 3	6
Weighted average remaining life					1
Regulatory liabilities other [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities					$ 341	$ 302
Weighted average remaining life					Various
Amount for discontinuance of CWIP in rate base [Member] | Alberta Utilities Commission [Member]
Regulatory Liabilities [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount			$ 82
Amount for change to flow through method for income taxes [Member] | Alberta Utilities Commission [Member]
Regulatory Liabilities [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount					$ 45
AltaLink [Member] | Alberta Utilities Commission [Member]
Regulatory Liabilities [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		$ 415
Public Utilities, Approved Rate Increase (Decrease), Amount					200
AltaLink [Member] | 2015-2016 GTA [Member] | GTA 2015 [Member] | Alberta Utilities Commission [Member]
Regulatory Liabilities [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount		599
AltaLink [Member] | 2015-2016 GTA [Member] | GTA 2016 [Member] | Alberta Utilities Commission [Member]
Regulatory Liabilities [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount		685
AltaLink [Member] | Amount previously collected as CWIP given as refund [Member] | Alberta Utilities Commission [Member]
Regulatory Liabilities [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount		277			$ 200
AltaLink [Member] | Amount previously collected as CWIP given as returns [Member] | Alberta Utilities Commission [Member]
Regulatory Liabilities [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount		12
AltaLink [Member] | Amount for change to flow through method for income taxes [Member] | Alberta Utilities Commission [Member]
Regulatory Liabilities [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount		$ 68
2015-2016 GTA [Member] | AltaLink [Member] | GTA 2015 [Member] | Alberta Utilities Commission [Member]
Regulatory Liabilities [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount			672	$ 811
2015-2016 GTA [Member] | AltaLink [Member] | GTA 2016 [Member] | Alberta Utilities Commission [Member]
Regulatory Liabilities [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount			$ 704	$ 1,000
Tax Cuts and Jobs Act of 2017 [Member]
Regulatory Liabilities [Line Items]
Federal statutory income tax rate					21.00%
Tax Cuts and Jobs Act of 2017 [Member] | Deferred Income Tax Charge [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	[4]				$ 5,950

[1] (1)Amounts primarily represent income tax liabilities related to the federal tax rate change from 35% to 21% that are probable to be passed on to customers, offset by income tax benefits related to certain property-related basis differences and other various differences that were previously passed on to customers and will be included in regulated rates when the temporary differences reverse. See Note 11 for further discussion of 2017 Tax Reform impacts.

[2] Amounts represent estimated costs, as accrued through depreciation rates and exclusive of ARO liabilities, of removing regulated property, plant and equipment in accordance with accepted regulatory practices. Amounts are deducted from rate base or otherwise accrue a carrying cost.

[3] (3)Represents amounts not yet recognized as a component of net periodic benefit cost that are to be returned to customers in future periods when recognized.

[4] (1)Amounts primarily represent income tax liabilities related to the federal tax rate change from 35% to 21% that are probable to be passed on to customers, offset by income tax benefits related to accelerated tax depreciation and certain property-related basis differences that were previously passed on to customers and will be included in regulated rates when the temporary differences reverse. See Note 10 for further discussion of 2017 Tax Reform impacts.