Allowance for credit losses - Interest Income Recognized on Loans (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Loans And Leases Receivable [Line Items]
Average Recorded Investment	$ 1,063,782	$ 1,145,129	$ 1,149,341
Interest Income Recognized, Total	29,419	25,028	25,046
Interest Income Recognized, Cash Basis	18,520	12,313	12,649
Commercial, Financial, Leasing, etc [Member]
Loans And Leases Receivable [Line Items]
Average Recorded Investment	151,314	163,485	244,687
Interest Income Recognized, Total	2,938	3,306	4,834
Interest Income Recognized, Cash Basis	2,938	3,278	4,810
Commercial Real Estate [Member]
Loans And Leases Receivable [Line Items]
Average Recorded Investment	185,171	189,837	240,123
Interest Income Recognized, Total	2,834	2,060	1,983
Interest Income Recognized, Cash Basis	2,834	1,985	1,740
Residential Builder and Developer [Member]
Loans And Leases Receivable [Line Items]
Average Recorded Investment	249,191	317,296	252,229
Interest Income Recognized, Total	1,563	1,948	1,716
Interest Income Recognized, Cash Basis	1,102	860	746
Other Commercial Construction [Member]
Loans And Leases Receivable [Line Items]
Average Recorded Investment	99,672	105,947	60,416
Interest Income Recognized, Total	5,020	926	371
Interest Income Recognized, Cash Basis	5,020	684	357
Residential [Member]
Loans And Leases Receivable [Line Items]
Average Recorded Investment	132,888	99,107	62,104
Interest Income Recognized, Total	5,284	4,271	3,028
Interest Income Recognized, Cash Basis	3,300	2,286	1,755
Residential Alt-A [Member]
Loans And Leases Receivable [Line Items]
Average Recorded Investment	171,546	196,161	220,589
Interest Income Recognized, Total	7,175	7,713	8,397
Interest Income Recognized, Cash Basis	2,226	1,965	1,758
Home equity lines and loans [Member]
Loans And Leases Receivable [Line Items]
Average Recorded Investment	11,322	11,428	11,807
Interest Income Recognized, Total	663	681	790
Interest Income Recognized, Cash Basis	179	106	202
Automobile [Member]
Loans And Leases Receivable [Line Items]
Average Recorded Investment	51,650	56,862	54,221
Interest Income Recognized, Total	3,470	3,850	3,684
Interest Income Recognized, Cash Basis	724	1,060	1,233
Other [Member]
Loans And Leases Receivable [Line Items]
Average Recorded Investment	11,028	5,006	3,165
Interest Income Recognized, Total	472	273	243
Interest Income Recognized, Cash Basis	$ 197	$ 89	$ 48