Segment information - Summary of Segment Information (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012
Sales Information [Line Items]
Segment Reporting Information Average Asset	$ 82,637	$ 79,983	$ 79,056
Business Banking [Member]
Sales Information [Line Items]
Segment Reporting Information Average Asset	5,010	4,909	4,932
Commercial Banking [Member]
Sales Information [Line Items]
Segment Reporting Information Average Asset	21,446	19,946	19,455
Commercial Real Estate [Member]
Sales Information [Line Items]
Segment Reporting Information Average Asset	17,122	16,437	16,316
Discretionary Portfolio [Member]
Sales Information [Line Items]
Segment Reporting Information Average Asset	16,278	16,583	16,136
Residential Mortgage Banking [Member]
Sales Information [Line Items]
Segment Reporting Information Average Asset	2,752	2,451	2,200
Retail Banking [Member]
Sales Information [Line Items]
Segment Reporting Information Average Asset	11,357	11,705	11,791
All Other [Member]
Sales Information [Line Items]
Segment Reporting Information Average Asset	$ 8,672	$ 7,952	$ 8,226