Financing Activities - Equity Securities (Details) $ / shares in Units, $ in Millions		1 Months Ended	3 Months Ended	6 Months Ended
		May 31, 2018 counterparties shares	Jun. 30, 2018 USD ($) shares	Jun. 30, 2018 USD ($) $ / shares shares	Jun. 30, 2017 USD ($) shares	Feb. 28, 2018 USD ($)
Equity Programs [Line Items]
Issuance of common stock | $				$ 147	$ 177
Common Stock [Member]
Equity Programs [Line Items]
Common stock shares issued	[1]			5,730,000	5,742,000
Forward Sale Agreement [Member] | Common Stock [Member]
Equity Programs [Line Items]
Forward Contract Indexed to Issuer's Equity, Shares		55,000,000
Number of Counterparties | counterparties		2
Total Number Of Shares Issued In Forward Sales Agreements And Overallotment Option		63,250,000.00
Sale of Stock, Price Per Share | $ / shares				$ 26.7057
Over-Allotment Option [Member] | Forward Sale Agreement [Member] | Common Stock [Member]
Equity Programs [Line Items]
Forward Contract Indexed to Issuer's Equity, Shares		8,250,000.00
Maximum [Member] | Forward Sale Agreement [Member] | Common Stock [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%
Common stock shares issued			1,200,000	4,200,000
Issuance of common stock | $			$ 34	$ 119

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