Loans and Allowance for Loan Losses - Composition of loan portfolio (Details) - USD ($) $ in Thousands		Mar. 31, 2018	Dec. 31, 2017
Loans and Leases Receivable Disclosure [Line Items]
Loans		$ 62,207,861	$ 61,623,768
Commercial, Financial and Agricultural
Loans and Leases Receivable Disclosure [Line Items]
Loans		26,359,127	25,749,949
Commercial Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Loans	[1]	13,685,143	13,997,697
Commercial Real Estate | Real estate – construction
Loans and Leases Receivable Disclosure [Line Items]
Loans		2,156,856	2,273,539
Commercial Real Estate | Commercial real estate – mortgage
Loans and Leases Receivable Disclosure [Line Items]
Loans		11,528,287	11,724,158
Residential Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Loans	[2]	16,309,384	16,382,116
Residential Real Estate | Residential real estate – mortgage
Loans and Leases Receivable Disclosure [Line Items]
Loans		13,306,987	13,365,747
Residential Real Estate | Equity lines of credit
Loans and Leases Receivable Disclosure [Line Items]
Loans		2,657,860	2,653,105
Residential Real Estate | Equity loans
Loans and Leases Receivable Disclosure [Line Items]
Loans		344,537	363,264
Consumer
Loans and Leases Receivable Disclosure [Line Items]
Loans	[3]	5,854,207	5,494,006
Consumer | Credit card
Loans and Leases Receivable Disclosure [Line Items]
Loans		657,030	639,517
Consumer | Consumer direct
Loans and Leases Receivable Disclosure [Line Items]
Loans		1,856,443	1,690,383
Consumer | Consumer indirect
Loans and Leases Receivable Disclosure [Line Items]
Loans		3,340,734	3,164,106
Commercial
Loans and Leases Receivable Disclosure [Line Items]
Loans		40,044,270	39,747,646
Commercial | Commercial, Financial and Agricultural
Loans and Leases Receivable Disclosure [Line Items]
Loans		26,359,127	25,749,949
Commercial | Commercial Real Estate | Real estate – construction
Loans and Leases Receivable Disclosure [Line Items]
Loans		2,156,856	2,273,539
Commercial | Commercial Real Estate | Commercial real estate – mortgage
Loans and Leases Receivable Disclosure [Line Items]
Loans		11,528,287	11,724,158
Consumer
Loans and Leases Receivable Disclosure [Line Items]
Loans		22,163,591	21,876,122
Consumer | Residential Real Estate | Residential real estate – mortgage
Loans and Leases Receivable Disclosure [Line Items]
Loans		13,306,987	13,365,747
Consumer | Residential Real Estate | Equity lines of credit
Loans and Leases Receivable Disclosure [Line Items]
Loans		2,657,860	2,653,105
Consumer | Residential Real Estate | Equity loans
Loans and Leases Receivable Disclosure [Line Items]
Loans		344,537	363,264
Consumer | Consumer | Credit card
Loans and Leases Receivable Disclosure [Line Items]
Loans		657,030	639,517
Consumer | Consumer | Consumer direct
Loans and Leases Receivable Disclosure [Line Items]
Loans		1,856,443	1,690,383
Consumer | Consumer | Consumer indirect
Loans and Leases Receivable Disclosure [Line Items]
Loans		$ 3,340,734	$ 3,164,106

[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.