Loans and Allowance for Loan Losses - Credit quality indicators (Details) - USD ($) $ in Thousands		Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Loans		$ 62,207,861	$ 61,623,768
Commercial, Financial and Agricultural
Financing Receivable, Recorded Investment [Line Items]
Loans		26,359,127	25,749,949
Commercial Real Estate
Financing Receivable, Recorded Investment [Line Items]
Loans	[1]	13,685,143	13,997,697
Commercial Real Estate | Real estate – construction
Financing Receivable, Recorded Investment [Line Items]
Loans		2,156,856	2,273,539
Commercial Real Estate | Commercial real estate – mortgage
Financing Receivable, Recorded Investment [Line Items]
Loans		11,528,287	11,724,158
Residential Real Estate
Financing Receivable, Recorded Investment [Line Items]
Loans	[2]	16,309,384	16,382,116
Residential Real Estate | Residential real estate – mortgage
Financing Receivable, Recorded Investment [Line Items]
Loans		13,306,987	13,365,747
Residential Real Estate | Equity lines of credit
Financing Receivable, Recorded Investment [Line Items]
Loans		2,657,860	2,653,105
Residential Real Estate | Equity loans
Financing Receivable, Recorded Investment [Line Items]
Loans		344,537	363,264
Consumer
Financing Receivable, Recorded Investment [Line Items]
Loans	[3]	5,854,207	5,494,006
Consumer | Credit card
Financing Receivable, Recorded Investment [Line Items]
Loans		657,030	639,517
Consumer | Consumer direct
Financing Receivable, Recorded Investment [Line Items]
Loans		1,856,443	1,690,383
Consumer | Consumer indirect
Financing Receivable, Recorded Investment [Line Items]
Loans		3,340,734	3,164,106
Commercial
Financing Receivable, Recorded Investment [Line Items]
Loans		40,044,270	39,747,646
Commercial | Commercial, Financial and Agricultural
Financing Receivable, Recorded Investment [Line Items]
Loans		26,359,127	25,749,949
Commercial | Commercial, Financial and Agricultural | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans		25,012,256	24,387,737
Commercial | Commercial, Financial and Agricultural | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans		561,816	614,006
Commercial | Commercial, Financial and Agricultural | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans		664,839	623,672
Commercial | Commercial, Financial and Agricultural | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans		120,216	124,534
Commercial | Commercial Real Estate | Real estate – construction
Financing Receivable, Recorded Investment [Line Items]
Loans		2,156,856	2,273,539
Commercial | Commercial Real Estate | Commercial real estate – mortgage
Financing Receivable, Recorded Investment [Line Items]
Loans		11,528,287	11,724,158
Commercial | Commercial Real Estate | Pass | Real estate – construction
Financing Receivable, Recorded Investment [Line Items]
Loans		2,133,741	2,257,659
Commercial | Commercial Real Estate | Pass | Commercial real estate – mortgage
Financing Receivable, Recorded Investment [Line Items]
Loans		11,143,691	11,309,484
Commercial | Commercial Real Estate | Special Mention | Real estate – construction
Financing Receivable, Recorded Investment [Line Items]
Loans		7,288	12,401
Commercial | Commercial Real Estate | Special Mention | Commercial real estate – mortgage
Financing Receivable, Recorded Investment [Line Items]
Loans		167,165	215,076
Commercial | Commercial Real Estate | Substandard | Real estate – construction
Financing Receivable, Recorded Investment [Line Items]
Loans		15,827	3,479
Commercial | Commercial Real Estate | Substandard | Commercial real estate – mortgage
Financing Receivable, Recorded Investment [Line Items]
Loans		202,099	187,049
Commercial | Commercial Real Estate | Doubtful | Real estate – construction
Financing Receivable, Recorded Investment [Line Items]
Loans		0	0
Commercial | Commercial Real Estate | Doubtful | Commercial real estate – mortgage
Financing Receivable, Recorded Investment [Line Items]
Loans		15,332	12,549
Consumer
Financing Receivable, Recorded Investment [Line Items]
Loans		22,163,591	21,876,122
Consumer | Residential Real Estate | Residential real estate – mortgage
Financing Receivable, Recorded Investment [Line Items]
Loans		13,306,987	13,365,747
Consumer | Residential Real Estate | Equity lines of credit
Financing Receivable, Recorded Investment [Line Items]
Loans		2,657,860	2,653,105
Consumer | Residential Real Estate | Equity loans
Financing Receivable, Recorded Investment [Line Items]
Loans		344,537	363,264
Consumer | Residential Real Estate | Performing | Residential real estate – mortgage
Financing Receivable, Recorded Investment [Line Items]
Loans		13,130,644	13,182,760
Consumer | Residential Real Estate | Performing | Equity lines of credit
Financing Receivable, Recorded Investment [Line Items]
Loans		2,619,139	2,616,825
Consumer | Residential Real Estate | Performing | Equity loans
Financing Receivable, Recorded Investment [Line Items]
Loans		332,619	350,531
Consumer | Residential Real Estate | Nonperforming | Residential real estate – mortgage
Financing Receivable, Recorded Investment [Line Items]
Loans		176,343	182,987
Consumer | Residential Real Estate | Nonperforming | Equity lines of credit
Financing Receivable, Recorded Investment [Line Items]
Loans		38,721	36,280
Consumer | Residential Real Estate | Nonperforming | Equity loans
Financing Receivable, Recorded Investment [Line Items]
Loans		11,918	12,733
Consumer | Consumer | Credit card
Financing Receivable, Recorded Investment [Line Items]
Loans		657,030	639,517
Consumer | Consumer | Consumer direct
Financing Receivable, Recorded Investment [Line Items]
Loans		1,856,443	1,690,383
Consumer | Consumer | Consumer indirect
Financing Receivable, Recorded Investment [Line Items]
Loans		3,340,734	3,164,106
Consumer | Consumer | Performing | Credit card
Financing Receivable, Recorded Investment [Line Items]
Loans		645,185	627,588
Consumer | Consumer | Performing | Consumer direct
Financing Receivable, Recorded Investment [Line Items]
Loans		1,844,303	1,681,246
Consumer | Consumer | Performing | Consumer indirect
Financing Receivable, Recorded Investment [Line Items]
Loans		3,321,621	3,147,223
Consumer | Consumer | Nonperforming | Credit card
Financing Receivable, Recorded Investment [Line Items]
Loans		11,845	11,929
Consumer | Consumer | Nonperforming | Consumer direct
Financing Receivable, Recorded Investment [Line Items]
Loans		12,140	9,137
Consumer | Consumer | Nonperforming | Consumer indirect
Financing Receivable, Recorded Investment [Line Items]
Loans		$ 19,113	$ 16,883

[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.