Financing Activities Financing Activities- Equity Securities (Details) $ / shares in Units, $ in Millions		1 Months Ended		12 Months Ended
		Sep. 30, 2018 USD ($) shares	May 31, 2018 counterparties $ / shares shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) shares	Feb. 28, 2018 USD ($)
Equity Programs [Line Items]
Issuance of common stock | $				$ 698	$ 453	$ 144
Common Stock [Member]
Equity Programs [Line Items]
Common stock shares issued	[1]			26,925,000	13,667,000	5,874,000
Common Stock [Member] | Forward Sale Agreement [Member]
Equity Programs [Line Items]
Issuance of common stock | $		$ 520
Common stock shares issued		20,000,000
Sale of stock, price per share | $ / shares			$ 26.7057
Total Number Of Shares Issued In Forward Sales Agreements And Overallotment Option			63,250,000.00
Number of Counterparties | counterparties			2
Forward Contract Indexed to Issuer's Equity, Shares			55,000,000
Over-Allotment Option [Member] | Common Stock [Member] | Forward Sale Agreement [Member]
Equity Programs [Line Items]
Forward Contract Indexed to Issuer's Equity, Shares			8,250,000.00
Maximum [Member] | Common Stock [Member] | Forward Sale Agreement [Member]
Equity Programs [Line Items]
Forward Contract Indexed to Issuer's Equity, Settlement Date			Nov. 30, 2019
At The Market Stock Offering Program [Member]
Equity Programs [Line Items]
Common Stock Aggregate Sales Price | $							$ 1,000
Maximum Percentage Paid To Selling Agents				2.00%
Issuance of common stock | $				$ 119
Common stock shares issued				4,200,000

[1] Shares in thousands. Each share entitles the holder to one vote on any question presented at any shareowners' meeting.