Derivative Instruments and Hedging Activities - Schedule of Notional Amounts and Gross Fair Values of Derivative Assets and Derivative Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Derivatives, Fair Value [Line Items]
Notional amount	$ 12,480.0		$ 9,558.0
Derivative asset, fair value, amount not offset against collateral	3.0		1.0
Derivative liability, fair value, amount not offset against collateral	37.0		23.0
Derivative asset, not subject to master netting arrangement	373.0		153.0
Derivative liability, not subject to master netting arrangement	3.0		2.0
Derivative asset	376.0		154.0
Derivative liabilities	40.0		25.0
Interest rate contracts
Derivatives, Fair Value [Line Items]
Notional amount		$ 225.0
Derivative asset, fair value, amount not offset against collateral	3.0		1.0
Derivative liability, fair value, amount not offset against collateral	0.0		0.0
Interest Rate Swap
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, amount not offset against collateral	0.0		0.0
Derivative liability, fair value, amount not offset against collateral	37.0		23.0
Derivative asset, not subject to master netting arrangement	349.0		149.0
Derivative liability, not subject to master netting arrangement	0.0		1.0
Interest Rate Lock Commitments
Derivatives, Fair Value [Line Items]
Derivative asset, not subject to master netting arrangement	24.0		3.0
Derivative liability, not subject to master netting arrangement	0.0		0.0
Forward Delivery Commitments
Derivatives, Fair Value [Line Items]
Derivative asset, not subject to master netting arrangement	0.0		1.0
Derivative liability, not subject to master netting arrangement	2.0		1.0
Credit Risk Contracts
Derivatives, Fair Value [Line Items]
Notional amount	294.4
Derivative asset, not subject to master netting arrangement	0.0		0.0
Derivative liability, not subject to master netting arrangement	1.0		0.0
Subject to Master Netting Arrangement
Derivatives, Fair Value [Line Items]
Notional amount	6,221.0		5,295.0
Subject to Master Netting Arrangement \| Interest rate contracts
Derivatives, Fair Value [Line Items]
Notional amount	1,430.0		1,655.0
Subject to Master Netting Arrangement \| Interest Rate Swap
Derivatives, Fair Value [Line Items]
Notional amount	4,791.0		3,640.0
Not Subject to Master Netting Arrangement
Derivatives, Fair Value [Line Items]
Notional amount	6,259.0		4,263.0
Not Subject to Master Netting Arrangement \| Interest Rate Swap
Derivatives, Fair Value [Line Items]
Notional amount	4,791.0		3,640.0
Not Subject to Master Netting Arrangement \| Interest Rate Lock Commitments
Derivatives, Fair Value [Line Items]
Notional amount	531.0		163.0
Not Subject to Master Netting Arrangement \| Forward Delivery Commitments
Derivatives, Fair Value [Line Items]
Notional amount	500.0		195.0
Not Subject to Master Netting Arrangement \| Credit Risk Contracts
Derivatives, Fair Value [Line Items]
Notional amount	$ 437.0		$ 265.0

Fair value of financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, not subject to a master netting arrangement.

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