Business Acquisitions (Details) $ in Millions, CAD in Billions	1 Months Ended	9 Months Ended
	Dec. 31, 2014 USD ($) substation mi	Sep. 30, 2015 USD ($)	Dec. 01, 2014 USD ($)	Dec. 01, 2014 CAD
Business Acquisition [Line Items]
Goodwill	$ 9,343	$ 9,143
Business acquisition, measurement period		12
AltaLink, LP Transaction [Member]
Business Acquisition [Line Items]
Current assets, including cash and cash equivalents of $15			$ 174
Property, plant and equipment			5,610
Goodwill			1,731
Other long-term assets			128
Total assets			7,643
Current liabilities, including current portion of long-term debt of $79			866
Subsidiary debt, less current portion			3,772
Deferred income taxes			95
Other long-term liabilities			182
Total liabilities			4,915
Net assets acquired			2,728	CAD 3.1
Cash and cash equivalents assumed			15
Current portion of long-term debt assumed			$ 79
Transmission line | mi	7,800
Substation | substation	300
Common Stock [Member] | AltaLink, LP Transaction [Member]
Business Acquisition [Line Items]
Business acquisition, percentage of voting interests acquired			100.00%	100.00%
Junior Subordinated Debt [Member] | AltaLink, LP Transaction [Member]
Business Acquisition [Line Items]
Payments to acquire businesses, gross	$ 1,500
Commercial Paper [Member] | AltaLink, LP Transaction [Member]
Business Acquisition [Line Items]
Payments to acquire businesses, gross	$ 1,000