Equity Equity (Preferred Stock Classification) (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended			6 Months Ended
	Jan. 15, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Class of Stock [Line Items]
Treasury Stock, Value, Acquired, Cost Method		$ 500	$ 750	$ 500	$ 500	$ 1,300
Preferred Stock, Shares Authorized					200,000,000		200,000,000
Preferred Stock, Shares Issued					26,072,200		26,032,200
Preferred Stock, Shares Outstanding					26,072,200		26,032,200
Preferred Stock, Par or Stated Value Per Share					$ 0.01		$ 0.01
Proceeds from Issuance of Preferred Stock and Preference Stock		$ 972			$ 972	$ 0
Series A Preferred Stock
Class of Stock [Line Items]
Preferred Stock, Shares Authorized					27,600,000		27,600,000
Preferred Stock, Shares Issued					24,000,000		24,000,000
Preferred Stock, Shares Outstanding					24,000,000		24,000,000
Series C Preferred Stock
Class of Stock [Line Items]
Preferred Stock, Shares Authorized					1,500,000		1,500,000
Preferred Stock, Shares Issued					1,500,000		1,500,000
Preferred Stock, Shares Outstanding					1,500,000		1,500,000
Series D Preferred Stock
Class of Stock [Line Items]
Preferred Stock, Shares Authorized					500,000		500,000
Preferred Stock, Shares Issued					500,000		500,000
Preferred Stock, Shares Outstanding					500,000		500,000
Series E Preferred Stock
Class of Stock [Line Items]
Preferred Stock, Shares Authorized					32,200		32,200
Preferred Stock, Shares Issued					32,200		32,200
Preferred Stock, Shares Outstanding					32,200		32,200
Series F Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Shares Authorized					40,000		0
Preferred Stock, Shares Issued	40,000				40,000		0
Preferred Stock, Shares Outstanding					40,000		0
Preferred Stock, Dividend Rate, Percentage	4.75%
Preferred Stock, Par or Stated Value Per Share	$ 0.01
Preferred Stock, Liquidation Preference Per Share	$ 25,000
Proceeds from Issuance of Preferred Stock and Preference Stock	$ 972
Payments of Stock Issuance Costs	$ 28
Series A Junior Preferred Stock
Class of Stock [Line Items]
Preferred Stock, Shares Authorized					10,000,000		10,000,000
Preferred Stock, Shares Issued					0		0
Preferred Stock, Shares Outstanding					0		0
Not Designated Preferred Stock
Class of Stock [Line Items]
Preferred Stock, Shares Authorized					160,327,800		160,367,800
Preferred Stock, Shares Issued					0		0
Preferred Stock, Shares Outstanding					0		0
RatingAgency [Member] | Series F Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Liquidation Preference Per Share	$ 25,500