Loans and Allowance for Loan Losses - Composition of loan portfolio (Details) - USD ($) $ in Thousands		Sep. 30, 2020	Dec. 31, 2019
Loans and Leases Receivable Disclosure [Line Items]
Loans		$ 66,180,612	$ 63,946,857
Commercial, Financial and Agricultural
Loans and Leases Receivable Disclosure [Line Items]
Loans		26,940,173	24,432,238
Commercial Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Loans	[1]		15,890,160
Commercial Real Estate | Real estate – construction
Loans and Leases Receivable Disclosure [Line Items]
Loans		2,403,674	2,028,682
Commercial Real Estate | Commercial real estate – mortgage
Loans and Leases Receivable Disclosure [Line Items]
Loans		13,695,800	13,861,478
Residential Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Loans	[2]		16,371,602
Residential Real Estate | Residential real estate – mortgage
Loans and Leases Receivable Disclosure [Line Items]
Loans		13,463,757	13,533,954
Residential Real Estate | Equity lines of credit
Loans and Leases Receivable Disclosure [Line Items]
Loans		2,441,723	2,592,680
Residential Real Estate | Equity loans
Loans and Leases Receivable Disclosure [Line Items]
Loans		194,367	244,968
Consumer
Loans and Leases Receivable Disclosure [Line Items]
Loans	[3]		7,252,857
Consumer | Credit card
Loans and Leases Receivable Disclosure [Line Items]
Loans		907,793	1,002,365
Consumer | Consumer direct
Loans and Leases Receivable Disclosure [Line Items]
Loans		2,023,696	2,338,142
Consumer | Consumer indirect
Loans and Leases Receivable Disclosure [Line Items]
Loans		4,109,629	3,912,350
Commercial
Loans and Leases Receivable Disclosure [Line Items]
Loans		43,039,647	40,322,398
Commercial | Commercial, Financial and Agricultural
Loans and Leases Receivable Disclosure [Line Items]
Loans		26,940,173	24,432,238
Commercial | Commercial Real Estate | Real estate – construction
Loans and Leases Receivable Disclosure [Line Items]
Loans		2,403,674	2,028,682
Commercial | Commercial Real Estate | Commercial real estate – mortgage
Loans and Leases Receivable Disclosure [Line Items]
Loans		13,695,800	13,861,478
Consumer
Loans and Leases Receivable Disclosure [Line Items]
Loans		23,140,965	23,624,459
Consumer | Residential Real Estate | Residential real estate – mortgage
Loans and Leases Receivable Disclosure [Line Items]
Loans		13,463,757	13,533,954
Consumer | Residential Real Estate | Equity lines of credit
Loans and Leases Receivable Disclosure [Line Items]
Loans		2,441,723	2,592,680
Consumer | Residential Real Estate | Equity loans
Loans and Leases Receivable Disclosure [Line Items]
Loans		194,367	244,968
Consumer | Consumer | Credit card
Loans and Leases Receivable Disclosure [Line Items]
Loans		907,793	1,002,365
Consumer | Consumer | Consumer direct
Loans and Leases Receivable Disclosure [Line Items]
Loans		2,023,696	2,338,142
Consumer | Consumer | Consumer indirect
Loans and Leases Receivable Disclosure [Line Items]
Loans		$ 4,109,629	$ 3,912,350

[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.