Commitments and contingencies (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Commitments to extend credit
Home equity lines of credit	$ 6,399,753	$ 6,393,332
Commercial real estate loans to be sold	190,397	177,982
Other commercial real estate and construction	2,788,952	2,818,071
Residential real estate loans to be sold	295,169	182,474
Other residential real estate	243,465	129,466
Commercial and other	10,936,656	10,442,754
Standby letters of credit	3,870,093	3,930,271
Commercial letters of credit	54,096	44,981
Financial guarantees and indemnification contracts	2,245,496	1,903,254
Commitments to sell real estate loans	$ 602,652	$ 635,899