Segment information - Summary of Segment Information (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Operating Statistics [Line Items]
Segment Reporting Information Average Asset	$ 79,518	$ 72,489	$ 73,977
Business Banking [Member]
Operating Statistics [Line Items]
Segment Reporting Information Average Asset	4,918	4,995	5,192
Commercial Banking [Member]
Operating Statistics [Line Items]
Segment Reporting Information Average Asset	19,691	17,397	17,650
Commercial Real Estate [Member]
Operating Statistics [Line Items]
Segment Reporting Information Average Asset	16,364	14,730	15,025
Discretionary Portfolio [Member]
Operating Statistics [Line Items]
Segment Reporting Information Average Asset	16,428	13,844	14,170
Residential Mortgage Banking [Member]
Operating Statistics [Line Items]
Segment Reporting Information Average Asset	2,316	1,913	1,958
Retail Banking [Member]
Operating Statistics [Line Items]
Segment Reporting Information Average Asset	11,746	11,904	11,940
All Other [Member]
Operating Statistics [Line Items]
Segment Reporting Information Average Asset	$ 8,055	$ 7,706	$ 8,042