Shareholders' Equity - Additional Information (Detail) $ / shares in Units, shares in Thousands	1 Months Ended			3 Months Ended			12 Months Ended
	Oct. 31, 2021 USD ($) shares	Jul. 31, 2021 shares	May 31, 2020 USD ($) shares	Sep. 30, 2021 USD ($) transaction $ / shares shares	Jun. 30, 2021 transaction shares	Dec. 31, 2020 USD ($)	Sep. 30, 2021 USD ($) lawsuit transaction $ / shares shares	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Nov. 04, 2021 shares	May 01, 2020 shares
Class of Stock [Line Items]
Stock Repurchase Program, Number of Shares Authorized to be Repurchased				10,000			10,000
Other comprehensive loss before reclassifications, net of taxes | $							$ 383,000,000	$ (338,000,000)	$ (427,000,000)
Number of Accelerated Share Repurchase Agreements | lawsuit							2
Subsequent Event
Class of Stock [Line Items]
Stock Repurchase Program, Number of Shares Authorized to be Repurchased										10,000
Treasury Stock
Class of Stock [Line Items]
Treasury Stock, Shares, Acquired							6,643
Treasury Stock, Value, Acquired, Cost Method | $							$ 1,600,000,000
Benefit Plans
Class of Stock [Line Items]
Other comprehensive loss before reclassifications, net of taxes | $						$ 24,000,000	$ 187,000,000	$ (101,000,000)	$ (325,000,000)
Open Market Repurchases
Class of Stock [Line Items]
Treasury Stock, Shares, Acquired							2,066
Open Market Repurchases | Treasury Stock
Class of Stock [Line Items]
Treasury Stock, Value, Acquired, Cost Method | $							$ 500,000,000
Accelerated Share Repurchase Program
Class of Stock [Line Items]
Number of Transactions | transaction				2	2		2
Treasury Stock, Shares, Acquired		403		2,515	1,658
Accelerated Share Repurchase Program | Subsequent Event
Class of Stock [Line Items]
Treasury Stock, Shares, Acquired	462
Accelerated Share Repurchase Program | Treasury Stock | Subsequent Event
Class of Stock [Line Items]
Treasury Stock, Value, Acquired, Cost Method | $	$ 150,000,000
Minimum
Class of Stock [Line Items]
Convertible Preferred Stock, Shares Issued upon Conversion				5,200			5,200
Maximum
Class of Stock [Line Items]
Convertible Preferred Stock, Shares Issued upon Conversion				6,200			6,200
Common Stock  Issued at Par Value
Class of Stock [Line Items]
Convertible Preferred Stock, Shares Issued upon Conversion											11,703
Stock Issued During Period, Shares, New Issues			6,250
Gross Proceeds from Issuance of Equity | $			$ 1,500,000,000
Proceeds from Issuance of Preferred Stock and Preference Stock | $			$ 1,459,000,000
Convertible Preferred Stock
Class of Stock [Line Items]
Preferred Stock, Depositary Share Ownership Interest, Percentage			5.00%
Preferred Stock, Shares Outstanding											2,475
Stock Issued During Period, Shares, New Issues			1,500
Gross Proceeds from Issuance of Equity | $			$ 1,500,000,000
Proceeds from Issuance of Preferred Stock and Preference Stock | $			$ 1,459,000,000
Depositary Share Liquidation | $				$ 50			$ 50
Preferred Stock, Liquidation Preference Per Share | $ / shares				$ 1,000			$ 1,000
Preferred Stock, Dividend Rate, Percentage							6.00%