Equity Other Equity Activity (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Class of Stock [Line Items]
Proceeds from Issuance of Common Stock				$ 841	$ 464
2015 ATM Program, Life to Date Proceeds, Gross	$ 350			350
2015 ATM Program, capacity remaining	150			150
Availability Under ATM	$ 750			$ 750
Preferred Stock, Par or Stated Value Per Share	$ 0.01			$ 0.01		$ 0.01
2015 ATM, Aggregate Value of Common Stock	$ 500			$ 500
March 2018 Equity Financing [Member]
Class of Stock [Line Items]
Proceeds from Issuance of Common Stock		$ 841
Stock Issued During Period, Shares, New Issues		8
Common Stock [Member]
Class of Stock [Line Items]
Proceeds from Issuance of Common Stock	$ 0
Stock Issued During Period, Shares, New Issues	0		5	8	4