Commitments and contingencies - Company's Significant Commitments (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Commitments to extend credit
Home equity lines of credit	$ 6,282,615	$ 6,393,332
Commercial real estate loans to be sold	139,929	177,982
Other commercial real estate and construction	3,819,342	2,818,071
Residential real estate loans to be sold	1,550,441	182,474
Other residential real estate	445,268	129,466
Commercial and other	10,070,711	10,442,754
Standby letters of credit	4,025,329	3,930,271
Commercial letters of credit	53,201	44,981
Financial guarantees and indemnification contracts	2,120,361	1,903,254
Commitments to sell real estate loans	$ 2,625,408	$ 635,899