Loans and Allowance for Loan Losses - Impaired loans by loan class (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Financing Receivable, Impaired [Line Items]
Individually evaluated impaired loans with no recorded allowance, recorded investment	$ 184,266		$ 180,897
Individually evaluated impaired loans with no recorded allowance, unpaid principle balance	221,208		215,016
Individually evaluated impaired loans with a recorded allowance, recorded investment	332,817		388,397
Individually evaluated impaired loans with a recorded allowance, unpaid principle balance	358,954		408,045
Individually evaluated impaired loans with a recorded allowance, allowance	95,197		104,385
Individually evaluated impaired loans, average recorded investment	516,222	$ 831,383
Individually evaluated impaired loans, interest income recognized	1,539	1,918
Commercial, Financial and Agricultural
Financing Receivable, Impaired [Line Items]
Individually evaluated impaired loans with no recorded allowance, recorded investment	145,794		142,908
Individually evaluated impaired loans with no recorded allowance, unpaid principle balance	181,112		175,743
Individually evaluated impaired loans with a recorded allowance, recorded investment	110,368		164,772
Individually evaluated impaired loans with a recorded allowance, unpaid principle balance	127,306		175,512
Individually evaluated impaired loans with a recorded allowance, allowance	52,896		61,705
Individually evaluated impaired loans, average recorded investment	256,249	598,126
Individually evaluated impaired loans, interest income recognized	136	404
Commercial Real Estate | Real estate – construction
Financing Receivable, Impaired [Line Items]
Individually evaluated impaired loans with no recorded allowance, recorded investment	2,797		2,849
Individually evaluated impaired loans with no recorded allowance, unpaid principle balance	2,835		2,858
Individually evaluated impaired loans with a recorded allowance, recorded investment	9,157		130
Individually evaluated impaired loans with a recorded allowance, unpaid principle balance	9,157		130
Individually evaluated impaired loans with a recorded allowance, allowance	1,584		7
Individually evaluated impaired loans, average recorded investment	5,978	334
Individually evaluated impaired loans, interest income recognized	2	2
Commercial Real Estate | Commercial real estate – mortgage
Financing Receivable, Impaired [Line Items]
Individually evaluated impaired loans with no recorded allowance, recorded investment	35,675		35,140
Individually evaluated impaired loans with no recorded allowance, unpaid principle balance	37,261		36,415
Individually evaluated impaired loans with a recorded allowance, recorded investment	48,668		47,061
Individually evaluated impaired loans with a recorded allowance, unpaid principle balance	56,974		55,122
Individually evaluated impaired loans with a recorded allowance, allowance	9,984		9,857
Individually evaluated impaired loans, average recorded investment	83,733	49,265
Individually evaluated impaired loans, interest income recognized	211	284
Residential Real Estate | Residential real estate – mortgage
Financing Receivable, Impaired [Line Items]
Individually evaluated impaired loans with no recorded allowance, recorded investment	0		0
Individually evaluated impaired loans with no recorded allowance, unpaid principle balance	0		0
Individually evaluated impaired loans with a recorded allowance, recorded investment	107,217		117,751
Individually evaluated impaired loans with a recorded allowance, unpaid principle balance	107,217		117,751
Individually evaluated impaired loans with a recorded allowance, allowance	8,434		10,214
Individually evaluated impaired loans, average recorded investment	111,057	116,979
Individually evaluated impaired loans, interest income recognized	680	646
Residential Real Estate | Equity lines of credit
Financing Receivable, Impaired [Line Items]
Individually evaluated impaired loans with no recorded allowance, recorded investment	0		0
Individually evaluated impaired loans with no recorded allowance, unpaid principle balance	0		0
Individually evaluated impaired loans with a recorded allowance, recorded investment	18,410		19,183
Individually evaluated impaired loans with a recorded allowance, unpaid principle balance	18,415		19,188
Individually evaluated impaired loans with a recorded allowance, allowance	15,383		16,021
Individually evaluated impaired loans, average recorded investment	18,756	23,338
Individually evaluated impaired loans, interest income recognized	194	229
Residential Real Estate | Equity loans
Financing Receivable, Impaired [Line Items]
Individually evaluated impaired loans with no recorded allowance, recorded investment	0		0
Individually evaluated impaired loans with no recorded allowance, unpaid principle balance	0		0
Individually evaluated impaired loans with a recorded allowance, recorded investment	35,672		35,923
Individually evaluated impaired loans with a recorded allowance, unpaid principle balance	36,560		36,765
Individually evaluated impaired loans with a recorded allowance, allowance	4,099		4,378
Individually evaluated impaired loans, average recorded investment	35,701	40,570
Individually evaluated impaired loans, interest income recognized	303	344
Consumer | Credit card
Financing Receivable, Impaired [Line Items]
Individually evaluated impaired loans with no recorded allowance, recorded investment	0		0
Individually evaluated impaired loans with no recorded allowance, unpaid principle balance	0		0
Individually evaluated impaired loans with a recorded allowance, recorded investment	0		0
Individually evaluated impaired loans with a recorded allowance, unpaid principle balance	0		0
Individually evaluated impaired loans with a recorded allowance, allowance	0		0
Individually evaluated impaired loans, average recorded investment	0	0
Individually evaluated impaired loans, interest income recognized	0	0
Consumer | Consumer direct
Financing Receivable, Impaired [Line Items]
Individually evaluated impaired loans with no recorded allowance, recorded investment	0		0
Individually evaluated impaired loans with no recorded allowance, unpaid principle balance	0		0
Individually evaluated impaired loans with a recorded allowance, recorded investment	2,499		2,545
Individually evaluated impaired loans with a recorded allowance, unpaid principle balance	2,499		2,545
Individually evaluated impaired loans with a recorded allowance, allowance	2,036		1,254
Individually evaluated impaired loans, average recorded investment	3,851	707
Individually evaluated impaired loans, interest income recognized	11	6
Consumer | Consumer indirect
Financing Receivable, Impaired [Line Items]
Individually evaluated impaired loans with no recorded allowance, recorded investment	0		0
Individually evaluated impaired loans with no recorded allowance, unpaid principle balance	0		0
Individually evaluated impaired loans with a recorded allowance, recorded investment	826		1,032
Individually evaluated impaired loans with a recorded allowance, unpaid principle balance	826		1,032
Individually evaluated impaired loans with a recorded allowance, allowance	781		$ 949
Individually evaluated impaired loans, average recorded investment	897	2,064
Individually evaluated impaired loans, interest income recognized	$ 2	$ 3