Loans and Allowance for Loan Losses - Past due loans (Details) - USD ($) $ in Thousands		Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual		$ 647,210	$ 658,865
Total Past Due and Impaired		1,099,958	1,147,179
Not Past Due or Impaired		61,107,903	60,476,589
Total		62,207,861	61,623,768
Accruing TDRs, Total Loans		101,985	101,243
30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Past Due		214,832	240,663
60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Past Due		90,027	88,030
90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Past Due		45,904	58,378
Commercial, Financial and Agricultural
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual		284,699	310,059
Accruing TDRs		5,231	1,213
Total Past Due and Impaired		323,007	347,965
Not Past Due or Impaired		26,036,120	25,401,984
Total		26,359,127	25,749,949
Commercial, Financial and Agricultural | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Past Due		15,427	14,804
Commercial, Financial and Agricultural | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Past Due		12,230	3,753
Commercial, Financial and Agricultural | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Past Due		5,420	18,136
Commercial Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	[1]	13,685,143	13,997,697
Commercial Real Estate | Real estate – construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual		14,183	5,381
Accruing TDRs		129	101
Total Past Due and Impaired		16,920	19,405
Not Past Due or Impaired		2,139,936	2,254,134
Total		2,156,856	2,273,539
Commercial Real Estate | Real estate – construction | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Past Due		450	12,293
Commercial Real Estate | Real estate – construction | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Past Due		240	70
Commercial Real Estate | Real estate – construction | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Past Due		1,918	1,560
Commercial Real Estate | Commercial real estate – mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual		115,285	111,982
Accruing TDRs		4,182	4,155
Total Past Due and Impaired		133,468	130,807
Not Past Due or Impaired		11,394,819	11,593,351
Total		11,528,287	11,724,158
Commercial Real Estate | Commercial real estate – mortgage | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Past Due		8,552	10,473
Commercial Real Estate | Commercial real estate – mortgage | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Past Due		3,220	3,270
Commercial Real Estate | Commercial real estate – mortgage | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Past Due		2,229	927
Residential Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	[2]	16,309,384	16,382,116
Residential Real Estate | Residential real estate – mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual		169,778	173,843
Accruing TDRs		62,171	64,898
Total Past Due and Impaired		345,252	351,227
Not Past Due or Impaired		12,961,735	13,014,520
Total		13,306,987	13,365,747
Residential Real Estate | Residential real estate – mortgage | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Past Due		73,335	69,474
Residential Real Estate | Residential real estate – mortgage | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Past Due		33,993	34,440
Residential Real Estate | Residential real estate – mortgage | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Past Due		5,975	8,572
Residential Real Estate | Equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual		36,350	34,021
Accruing TDRs		236	237
Total Past Due and Impaired		55,631	55,029
Not Past Due or Impaired		2,602,229	2,598,076
Total		2,657,860	2,653,105
Residential Real Estate | Equity lines of credit | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Past Due		12,063	10,956
Residential Real Estate | Equity lines of credit | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Past Due		4,611	7,556
Residential Real Estate | Equity lines of credit | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Past Due		2,371	2,259
Residential Real Estate | Equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual		11,429	11,559
Accruing TDRs		29,546	30,105
Total Past Due and Impaired		46,832	47,486
Not Past Due or Impaired		297,705	315,778
Total		344,537	363,264
Residential Real Estate | Equity loans | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Past Due		4,495	4,170
Residential Real Estate | Equity loans | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Past Due		983	657
Residential Real Estate | Equity loans | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Past Due		379	995
Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	[3]	5,854,207	5,494,006
Consumer | Credit card
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual		0	0
Accruing TDRs		0	0
Total Past Due and Impaired		23,372	23,443
Not Past Due or Impaired		633,658	616,074
Total		657,030	639,517
Consumer | Credit card | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Past Due		6,438	6,710
Consumer | Credit card | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Past Due		5,089	4,804
Consumer | Credit card | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Past Due		11,845	11,929
Consumer | Consumer direct
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual		4,211	2,425
Accruing TDRs		490	534
Total Past Due and Impaired		38,118	36,457
Not Past Due or Impaired		1,818,325	1,653,926
Total		1,856,443	1,690,383
Consumer | Consumer direct | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Past Due		16,919	19,766
Consumer | Consumer direct | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Past Due		8,569	7,020
Consumer | Consumer direct | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Past Due		7,929	6,712
Consumer | Consumer indirect
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual		11,275	9,595
Accruing TDRs		0	0
Total Past Due and Impaired		117,358	135,360
Not Past Due or Impaired		3,223,376	3,028,746
Total		3,340,734	3,164,106
Consumer | Consumer indirect | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Past Due		77,153	92,017
Consumer | Consumer indirect | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Past Due		21,092	26,460
Consumer | Consumer indirect | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Past Due		$ 7,838	$ 7,288

[1] Includes commercial real estate – mortgage and real estate – construction loans.

[2] Includes residential real estate – mortgage, equity lines of credit and equity loans.

[3] Includes credit card, consumer direct and consumer indirect loans.