Stockholders' Equity (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Mar. 31, 2018
Preferred Stock, Liquidation Preference, Value	$ 1,650,000,000	$ 1,650,000,000
Purchases of common stock, value	34,000,000	23,000,000	$ 25,000,000
At the Market Stock Offering Program, aggregate value of common stock	750,000,000			$ 500,000,000
Life to Date Net Proceeds Under ATM				350,000,000
Availability Under ATM	750,000,000			$ 150,000,000
Net proceeds from issuance of preferred stock	$ 0	$ 1,608,000,000	$ 0
Minimum [Member]
Convertible Preferred Stock, Shares Issued upon Conversion	8.7260
Maximum [Member]
Convertible Preferred Stock, Shares Issued upon Conversion	10.4712
March 2018 Equity Financing [Member]
Stock Issued During Period, Shares, New Issues	8,000,000
Proceeds from Issuance of Common Stock	$ 841,000,000
Preferred Stock [Member]
Preferred Stock, Liquidation Preference, Value	$ 1,000
Preferred Stock, Redemption Date	Aug. 01, 2020
Preferred Stock, Dividend Rate, Percentage	6.875%
Preferred Stock [Member] | July 2017 Equity Offering [Member]
Stock Issued During Period, Shares, New Issues	1,650,000
Sale of Stock, Transaction Date	Jul. 26, 2017
Preferred Stock, Dividend Rate, Percentage	6.875%
Preferred Stock, Per Share Sales Price	$ 1,000
Net proceeds from issuance of preferred stock	$ 1,608,000,000
Common Stock [Member]
Stock Issued During Period, Shares, New Issues	8,000,000	44,000,000	15,000,000
Proceeds from Issuance of Common Stock			$ 0
Purchases of common stock, shares	300,000
Common Stock [Member] | March 2018 Equity Financing [Member]
Sale of Stock, Transaction Date	Mar. 01, 2018
Common Stock [Member] | May 2017 Equity Offering [Member]
Stock Issued During Period, Shares, New Issues	4,750,000
Proceeds from Issuance of Common Stock	$ 442,000,000
Sale of Stock, Transaction Date	May 01, 2017
Common Stock [Member] | July 2017 Equity Offering [Member]
Stock Issued During Period, Shares, New Issues	40,150,000
Proceeds from Issuance of Common Stock	$ 3,800,000,000
Sale of Stock, Transaction Date	Jul. 26, 2017