Loans and Allowance for Credit Losses Allowance for Loan Losses by Portfolio Segment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning Balance	$ 169,910	$ 168,679
Loans charged off	(6,397)	(9,407)
Recoveries of loans previously charged off	2,362	5,929
Net loans charged off	(4,035)	(3,478)
Provision for loan losses	(2,658)	4,875
Ending Balance	163,217	170,076
Provision for loan losses gross	6,600	75
Provision for credit losses	3,970	4,800
Real-estate - commercial mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning Balance	58,793	46,842
Loans charged off	(267)	(1,224)
Recoveries of loans previously charged off	279	450
Net loans charged off	12	(774)
Provision for loan losses	(88)	1,305
Ending Balance	58,717	47,373
Commercial - industrial, financial and agricultural
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning Balance	66,280	54,353
Loans charged off	(4,005)	(5,527)
Recoveries of loans previously charged off	1,075	4,191
Net loans charged off	(2,930)	(1,336)
Provision for loan losses	(1,520)	2,292
Ending Balance	61,830	55,309
Real-estate - home equity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning Balance	18,127	26,801
Loans charged off	(408)	(698)
Recoveries of loans previously charged off	206	137
Net loans charged off	(202)	(561)
Provision for loan losses	(397)	(2,419)
Ending Balance	17,528	23,821
Real estate - residential mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning Balance	16,088	22,929
Loans charged off	(162)	(216)
Recoveries of loans previously charged off	107	230
Net loans charged off	(55)	14
Provision for loan losses	(772)	(925)
Ending Balance	15,261	22,018
Real-estate - construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning Balance	6,620	6,455
Loans charged off	(158)	(247)
Recoveries of loans previously charged off	306	548
Net loans charged off	148	301
Provision for loan losses	(844)	745
Ending Balance	5,924	7,501
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning Balance	2,045	3,574
Loans charged off	(892)	(856)
Recoveries of loans previously charged off	179	236
Net loans charged off	(713)	(620)
Provision for loan losses	571	77
Ending Balance	1,903	3,031
Leasing, other and overdrafts
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning Balance	1,957	3,192
Loans charged off	(505)	(639)
Recoveries of loans previously charged off	210	137
Net loans charged off	(295)	(502)
Provision for loan losses	392	578
Ending Balance	2,054	3,268
Unallocated
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning Balance	0	4,533
Loans charged off	0	0
Recoveries of loans previously charged off	0	0
Net loans charged off	0	0
Provision for loan losses	0	3,222
Ending Balance	$ 0	$ 7,755