Stockholders' Equity (Follow-On Equity Offerings) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions						12 Months Ended
	Nov. 19, 2018	Aug. 03, 2018	May 29, 2018	Sep. 12, 2017	May 05, 2017	Dec. 31, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Sale of Stock, Price Per Share	$ 17.09	$ 18.68	$ 18.35	$ 20.47	$ 20.51	$ 18.03	$ 20.96
Stock Issued During Period, Shares, New Issues	46.0	43.7	34.5	28.2	24.5
Stock Issued During Period, Value, New Issues	$ 786	$ 817	$ 633	$ 577	$ 503	$ 2,611	$ 1,238
Follow-On Public Offering [Member]
Class of Stock [Line Items]
Stock Issued During Period, Shares, New Issues						124.2	52.7
Stock Issued During Period, Value, New Issues						$ 2,236	$ 1,080