Basis of presentation (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011	Dec. 31, 2010
Consolidated Balance Sheet
Other assets	$ 566,734	$ 519,296
Total assets	9,821,957	9,594,477
Other liabilities	480,156	521,979
Total liabilities	8,211,888	8,031,478
Retained earnings	215,626	198,397		178,667
Total shareholders' equity	1,575,776	1,528,706	1,508,964	1,480,394
Total liabilities and shareholders' equity	9,821,957	9,594,477
Consolidated Statement of Changes in Shareholders' Equity
Retained earnings	215,626	198,397		178,667
Total shareholders' equity	1,575,776	1,528,706	1,508,964	1,480,394
As previously filed
Consolidated Balance Sheet
Other assets		517,550
Total assets		9,592,731
Other liabilities		516,990
Total liabilities		8,026,489
Retained earnings		201,640		181,910
Total shareholders' equity		1,531,949		1,483,637
Total liabilities and shareholders' equity		9,592,731
Consolidated Statement of Changes in Shareholders' Equity
Retained earnings		201,640		181,910
Total shareholders' equity		1,531,949		1,483,637
Difference
Consolidated Balance Sheet
Other assets		1,746
Total assets		1,746
Other liabilities		4,989
Total liabilities		4,989
Retained earnings		(3,243)		(3,243)
Total shareholders' equity		(3,243)		(3,243)
Total liabilities and shareholders' equity		1,746
Consolidated Statement of Changes in Shareholders' Equity
Retained earnings		(3,243)		(3,243)
Total shareholders' equity		$ (3,243)		$ (3,243)