Business Acquisitions (Details)	3 Months Ended						1 Months Ended
	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2014 AltaLink, LP Transaction [Member] mi substation	Dec. 01, 2014 AltaLink, LP Transaction [Member] USD ($)	Dec. 01, 2014 AltaLink, LP Transaction [Member] CAD	Dec. 01, 2014 Common Stock [Member] AltaLink, LP Transaction [Member]	Dec. 31, 2014 Junior Subordinated Debt [Member] AltaLink, LP Transaction [Member] USD ($)	Dec. 31, 2014 Commercial Paper [Member] AltaLink, LP Transaction [Member] USD ($)
Business Acquisition [Line Items]
Current assets, including cash and cash equivalents of $15				$ 174,000,000
Property, plant and equipment				5,610,000,000
Goodwill	9,186,000,000	9,343,000,000		1,731,000,000
Other long-term assets				128,000,000
Total assets				7,643,000,000
Current liabilities, including current portion of long-term debt of $79				866,000,000
Subsidiary debt, less current portion				3,772,000,000
Deferred income taxes				95,000,000
Other long-term liabilities				182,000,000
Total liabilities				4,915,000,000
Net assets acquired				2,728,000,000	3,100,000,000
Cash and cash equivalents assumed				15,000,000
Current portion of long-term debt assumed				79,000,000
Business acquisition, percentage of voting interests acquired						100.00%
Payments to acquire businesses, gross							$ 1,500,000,000	$ 1,000,000,000
Transmission line			7,800
Substation			300
Business acquisition, measurement period	12