Related Party Transactions - Summary of Related Party Transactions (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Income/(Expense):
Interest income	$ 785	$ 859	$ 2,428	$ 2,567
Interest expense	(275)	(238)	(796)	(773)
Net interest expense	510	621	1,632	1,794
Total affiliate income	63	49	165	151
Total support services from HSBC affiliates	362	412	1,100	1,092
Parent and Other Subsidiaries of Common Parent [Member]
Income/(Expense):
Interest income	12	16	38	46
Interest expense	(23)	(23)	(70)	(57)
Net interest expense	(11)	(7)	(32)	(11)
Residential mortgage banking revenue		3	3	7
Stock based compensation expense with HSBC	8	14	27	38
HSBC Finance [Member]
Income/(Expense):
Total affiliate income	6	2	8	6
Total support services from HSBC affiliates	5	8	22	27
HSBC Markets [Member]
Income/(Expense):
Total affiliate income	4	9	14	19
Total support services from HSBC affiliates	81	70	227	192
Other HSBC Affiliates [Member]
Income/(Expense):
Total affiliate income	17	13	59	53
Total support services from HSBC affiliates	39	55	140	148
Other HSBC Affiliates Income [Member]
Income/(Expense):
Total affiliate income	36	25	84	73
HSBC Technology and Services [Member]
Income/(Expense):
Total support services from HSBC affiliates	$ 237	$ 279	$ 711	$ 725