Shareholders' Equity - Schedule of Preferred Shares Outstanding (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended					9 Months Ended
	Apr. 30, 2016	May 31, 2015	Nov. 30, 2014	Feb. 28, 2014	Aug. 31, 2012	Sep. 30, 2018	Sep. 30, 2017
Class of Stock [Line Items]
Proceeds from issuance of preferred stock, net						$ 495	$ 0
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					$ 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				$ 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			$ 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		$ 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	$ 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
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
Proceeds from issuance of preferred stock, net						$ 494
Charles River Development | Series H Preferred Stock
Class of Stock [Line Items]
Proceeds from issuance of preferred stock, net						$ 500