Shareholders' Equity - Additional Information (Detail) - USD ($) $ / shares in Units, shares in Thousands	1 Months Ended	3 Months Ended	5 Months Ended
	May 31, 2017	Dec. 31, 2016	Sep. 30, 2017
Class of Stock [Line Items]
Treasury Stock, Shares, Acquired		1,300
Treasury Stock, Value, Acquired, Cost Method		$ 220,000,000
Proceeds from Issuance of Common Stock	$ 2,400,000,000
Common Stock Issued at Par Value
Class of Stock [Line Items]
Stock Issued During Period, Shares, New Issues	14,025
Gross Proceeds From Issuance Of Equity	$ 2,500,000,000
Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Depositary Share Ownership Interest, Percentage	5.00%
Stock Issued During Period, Shares, New Issues	2,475
Gross Proceeds From Issuance Of Equity	$ 2,500,000,000
Proceeds from Issuance of Preferred Stock and Preference Stock	$ 2,400,000,000
Preferred Stock, Dividend Rate, Percentage			6.125%
Preferred Stock, Liquidation Preference Per Share			$ 1,000
Depositary Share Liquidation Preference			$ 50
Minimum
Class of Stock [Line Items]
Convertible Preferred Stock, Shares Issued upon Conversion			11,700
Maximum
Class of Stock [Line Items]
Convertible Preferred Stock, Shares Issued upon Conversion			14,000