Subsequent Events Subsequent Event (Preferred Stock Issuance) (Details) - USD ($) $ / shares in Units, $ in Millions		1 Months Ended	12 Months Ended
	Jan. 15, 2020	Jun. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jun. 04, 2018
Subsequent Event [Line Items]
Preferred Stock, Shares Issued			26,032,200	26,032,200
Preferred stock, par value			$ 0.01	$ 0.01
Preferred stock issued, net of issuance costs			$ 0	$ 1,274	$ 0
Series E Preferred Stock [Member]
Subsequent Event [Line Items]
Preferred Stock, Shares Issued			32,200	32,200		32,200
Preferred Stock, Dividend Rate, Percentage		5.625%	5.625%
Preferred stock, par value						$ 0.01
Preferred Stock, Liquidation Preference Per Share			$ 25,000			$ 25,000
Preferred stock issued, net of issuance costs		$ 780
Payments of Stock Issuance Costs		$ 25
Subsequent Event [Member] | Series F Preferred Stock [Member]
Subsequent Event [Line Items]
Preferred Stock, Shares Issued	40,000
Preferred Stock, Dividend Rate, Percentage	4.75%
Preferred stock, par value	$ 0.01
Preferred Stock, Liquidation Preference Per Share	$ 25,000
Preferred stock issued, net of issuance costs	$ 972
Payments of Stock Issuance Costs	$ 28
RatingAgency [Member] | Series E Preferred Stock [Member]
Subsequent Event [Line Items]
Preferred Stock, Liquidation Preference Per Share			$ 25,500
RatingAgency [Member] | Subsequent Event [Member] | Series F Preferred Stock [Member]
Subsequent Event [Line Items]
Preferred Stock, Liquidation Preference Per Share	$ 25,500