Commitments and contingencies - Company's Significant Commitments (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Commitments to extend credit
Home equity lines of credit	$ 6,319,441	$ 6,393,332
Commercial real estate loans to be sold	282,162	177,982
Other commercial real estate and construction	3,578,153	2,818,071
Residential real estate loans to be sold	1,645,005	182,474
Other residential real estate	475,989	129,466
Commercial and other	10,315,662	10,442,754
Standby letters of credit	3,920,218	3,930,271
Commercial letters of credit	46,188	44,981
Financial guarantees and indemnification contracts	1,994,704	1,903,254
Commitments to sell real estate loans	$ 2,337,536	$ 635,899