Loans and Allowance for Loan Losses - Average balances of impaired loans and income recognized on impaired loans (Details 2) (Loans Receivable, USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Financing Receivable, Allowance for Credit Losses [Line Items]
Average of individually impaired loans during year	$ 235,815	$ 280,626
Interest income recognized during impairment	1,826	2,267
Cash-basis interest income recognized	2,518	3,192
Residential Mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Average of individually impaired loans during year	20,414	24,262
Interest income recognized during impairment	241	225
Cash-basis interest income recognized	223	261
Residential construction
Financing Receivable, Allowance for Credit Losses [Line Items]
Average of individually impaired loans during year	40,592	54,467
Interest income recognized during impairment	326	401
Cash-basis interest income recognized	428	518
Consumer installment
Financing Receivable, Allowance for Credit Losses [Line Items]
Average of individually impaired loans during year	276	338
Interest income recognized during impairment	6	5
Cash-basis interest income recognized	6	5
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Average of individually impaired loans during year	174,533	201,559
Interest income recognized during impairment	1,253	1,636
Cash-basis interest income recognized	1,861	2,408
Commercial | Commercial (Secured By Real Estate)
Financing Receivable, Allowance for Credit Losses [Line Items]
Average of individually impaired loans during year	91,631	117,551
Interest income recognized during impairment	946	1,251
Cash-basis interest income recognized	1,000	1,341
Commercial | Commercial & industrial
Financing Receivable, Allowance for Credit Losses [Line Items]
Average of individually impaired loans during year	43,694	43,249
Interest income recognized during impairment	156	118
Cash-basis interest income recognized	629	610
Commercial | Commercial construction
Financing Receivable, Allowance for Credit Losses [Line Items]
Average of individually impaired loans during year	39,208	40,759
Interest income recognized during impairment	151	267
Cash-basis interest income recognized	$ 232	$ 457