Segment information - Summary of Segment Information (Detail) (Reporting Segment [Member], USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Sales Information [Line Items]
Average Assets	$ 83,100	$ 79,983	$ 79,518
Business Banking [Member]
Sales Information [Line Items]
Average Assets	5,041	4,909	4,918
Commercial Banking [Member]
Sales Information [Line Items]
Average Assets	21,554	19,946	19,691
Commercial Real Estate [Member]
Sales Information [Line Items]
Average Assets	17,112	16,437	16,364
Discretionary Portfolio [Member]
Sales Information [Line Items]
Average Assets	16,224	16,583	16,428
Residential Mortgage Banking [Member]
Sales Information [Line Items]
Average Assets	2,783	2,451	2,316
Retail Banking [Member]
Sales Information [Line Items]
Average Assets	11,304	11,705	11,746
All Other [Member]
Sales Information [Line Items]
Average Assets	$ 9,082	$ 7,952	$ 8,055