Shareholders' Equity - Schedule of Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended						9 Months Ended
	Sep. 30, 2018	Apr. 30, 2016	May 31, 2015	Nov. 30, 2014	Feb. 28, 2014	Aug. 31, 2012	Sep. 30, 2019	Sep. 30, 2018
Class of Stock [Line Items]
Proceeds from issuance of preferred stock, net of issuance costs							$ 0	$ 495
Series C Preferred Stock, Depository Share
Class of Stock [Line Items]
Depositary shares issued (shares)						20,000,000
Liquidation preference per share (USD per share)						$ 25
Series C Preferred Stock
Class of Stock [Line Items]
Ownership Interest Per Depositary Share						0.025%
Liquidation preference per share (USD per share)						$ 100,000
Proceeds from issuance of preferred stock, net of issuance costs						$ 488
Series D Preferred Stock, Depository Share
Class of Stock [Line Items]
Depositary shares issued (shares)					30,000,000
Liquidation preference per share (USD per share)					$ 25
Series D Preferred Stock
Class of Stock [Line Items]
Ownership Interest Per Depositary Share					0.025%
Liquidation preference per share (USD per share)					$ 100,000
Proceeds from issuance of preferred stock, net of issuance costs					$ 742
Series E Preferred Stock, Depository Share
Class of Stock [Line Items]
Depositary shares issued (shares)				30,000,000
Liquidation preference per share (USD per share)				$ 25
Series E Preferred Stock
Class of Stock [Line Items]
Ownership Interest Per Depositary Share				0.025%
Liquidation preference per share (USD per share)				$ 100,000
Proceeds from issuance of preferred stock, net of issuance costs				$ 728
Series F Preferred Stock, Depository Share
Class of Stock [Line Items]
Depositary shares issued (shares)			750,000
Liquidation preference per share (USD per share)			$ 1,000
Series F Preferred Stock
Class of Stock [Line Items]
Ownership Interest Per Depositary Share			1.00%
Liquidation preference per share (USD per share)			$ 100,000
Proceeds from issuance of preferred stock, net of issuance costs			$ 742
Series G Preferred Stock, Depository Share
Class of Stock [Line Items]
Depositary shares issued (shares)		20,000,000
Liquidation preference per share (USD per share)		$ 25
Series G Preferred Stock
Class of Stock [Line Items]
Ownership Interest Per Depositary Share		0.025%
Liquidation preference per share (USD per share)		$ 100,000
Proceeds from issuance of preferred stock, net of issuance costs		$ 493
Series H Preferred Stock, Depository Share
Class of Stock [Line Items]
Depositary shares issued (shares)	500,000
Liquidation preference per share (USD per share)	$ 1,000							$ 1,000
Series H Preferred Stock
Class of Stock [Line Items]
Ownership Interest Per Depositary Share	1.00%
Liquidation preference per share (USD per share)	$ 100,000							$ 100,000
Proceeds from issuance of preferred stock, net of issuance costs	$ 494