Litigation and Regulatory Matters - Additional Information (Details)	1 Months Ended	3 Months Ended	6 Months Ended
	Aug. 31, 2016 former_employee	Jun. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2014 trust
Loss Contingencies [Line Items]
Expense related to various legal matters		$ 16,000,000	$ 510,000,000
Estimate of reasonably possible loss		325,000,000	325,000,000
Compensatory damages sought			$ 37,000,000
Mortgage Securitization Activity Related Litigation [Member]
Loss Contingencies [Line Items]
Litigation Settlement, Amount Awarded to Other Party		492,000,000
Mortgage Securitization Pool Trust Litigation [Member]
Loss Contingencies [Line Items]
Number of identified trusts | trust				280
HSBC Holdings plc [Member] | Foreign Exchange Market Regulatory Investigation [Member]
Loss Contingencies [Line Items]
Number of former employees indicted | former_employee	2
Number of former employees found guilty | former_employee	1
HSBC North America [Member] | Mortgage Securitization Activity Related Litigation [Member]
Loss Contingencies [Line Items]
Litigation Settlement, Amount Awarded to Other Party		765,000,000
HSBC North America [Member] | Massachusetts State Attorney General Civil Investigation into RMBS Origination and Securitization Activities [Member]
Loss Contingencies [Line Items]
Litigation Settlement, Amount Awarded to Other Party		$ 26,800,000