Commitments and contingencies - Company's Significant Commitments (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Commitments to extend credit
Home equity lines of credit	$ 6,302,801	$ 6,282,615
Commercial real estate loans to be sold	105,976	139,929
Other commercial real estate and construction	4,017,585	3,819,342
Residential real estate loans to be sold	1,266,065	1,550,441
Other residential real estate	453,051	445,268
Commercial and other	9,782,222	10,070,711
Standby letters of credit	3,974,118	4,025,329
Commercial letters of credit	43,388	53,201
Financial guarantees and indemnification contracts	2,230,375	2,120,361
Commitments to sell real estate loans	$ 2,136,984	$ 2,625,408