Guarantee Arrangements, Pledged Assets and Repurchase Agreements - Additional Information (Detail) - USD ($) $ in Millions		3 Months Ended		36 Months Ended
		Sep. 30, 2019	Jun. 30, 2018	Dec. 31, 2007	Dec. 31, 2018
Guarantor Obligations [Line Items]
Carrying value of swap		$ 5,617,003			$ 5,556,668
Deferred fees		2			2
Pledged assets not separately reported, trading assets	[1]	4,802			2,465
Collateral accepted under security resale agreements, fair value		11,726			13,004
Collateral accepted under security repurchase agreements but not reported on the balance sheet that can be sold or repledged, fair value		10,776			11,904
Collateral accepted under security repurchase agreements that has been sold or repledged under repurchase agreements or to cover short sales		2,486			243
HSBC Securities [Member]
Guarantor Obligations [Line Items]
Whole loan securitization				$ 24,000
Securities available-for-sale [Member]
Guarantor Obligations [Line Items]
Securities pledged as collateral that can be sold or repledged by the secured party, fair value		1,780			822
Trading assets [Member]
Guarantor Obligations [Line Items]
Pledged assets not separately reported, trading assets		4,802			2,464
Obligation to Repurchase Receivables Sold [Member]
Guarantor Obligations [Line Items]
Outstanding repurchase demands		3			2
Other liabilities [Member]
Guarantor Obligations [Line Items]
Allowance for credit losses on unfunded standby letters of credit		19			16
Other liabilities [Member] | Standby Letters of Credit [Member]
Guarantor Obligations [Line Items]
Deferred fees		48			43
Swap [Member]
Guarantor Obligations [Line Items]
Carrying value of swap		38			40
Mortgage Securitization Activity Related Litigation [Member] | HSBC Securities [Member]
Guarantor Obligations [Line Items]
Mortgage securitization outstanding loan purchase and sold		3,500			$ 3,800
Visa Class B Shares [Member]
Guarantor Obligations [Line Items]
Loss Contingency, Loss in Period		$ 4	$ 7

[1] Trading assets are primarily pledged against liabilities associated with repurchase agreements.