Loans and Allowance for Loan Losses - Composition of loan portfolio (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Loans and Leases Receivable Disclosure [Line Items]
Covered loans	$ 341,939	$ 359,334
Total loans	59,705,767	60,061,263
Commercial, Financial and Agricultural
Loans and Leases Receivable Disclosure [Line Items]
Loans	24,660,668	25,122,002
Commercial Real Estate | Real estate – construction
Loans and Leases Receivable Disclosure [Line Items]
Loans	2,227,405	2,125,316
Commercial Real Estate | Commercial real estate – mortgage
Loans and Leases Receivable Disclosure [Line Items]
Loans	11,398,821	11,210,660
Residential Real Estate | Residential real estate – mortgage
Loans and Leases Receivable Disclosure [Line Items]
Loans	13,117,776	13,259,994
Residential Real Estate | Equity lines of credit
Loans and Leases Receivable Disclosure [Line Items]
Loans	2,560,495	2,543,778
Residential Real Estate | Equity loans
Loans and Leases Receivable Disclosure [Line Items]
Loans	420,362	445,709
Consumer | Credit card
Loans and Leases Receivable Disclosure [Line Items]
Loans	570,311	604,881
Consumer | Consumer direct
Loans and Leases Receivable Disclosure [Line Items]
Loans	1,329,242	1,254,641
Consumer | Consumer indirect
Loans and Leases Receivable Disclosure [Line Items]
Loans	3,078,748	3,134,948
Commercial
Loans and Leases Receivable Disclosure [Line Items]
Loans	38,286,894	38,457,978
Commercial | Commercial, Financial and Agricultural
Loans and Leases Receivable Disclosure [Line Items]
Loans	24,660,668	25,122,002
Commercial | Commercial Real Estate | Real estate – construction
Loans and Leases Receivable Disclosure [Line Items]
Loans	2,227,405	2,125,316
Commercial | Commercial Real Estate | Commercial real estate – mortgage
Loans and Leases Receivable Disclosure [Line Items]
Loans	11,398,821	11,210,660
Consumer
Loans and Leases Receivable Disclosure [Line Items]
Loans	21,076,934	21,243,951
Consumer | Residential Real Estate | Residential real estate – mortgage
Loans and Leases Receivable Disclosure [Line Items]
Loans	13,117,776	13,259,994
Consumer | Residential Real Estate | Equity lines of credit
Loans and Leases Receivable Disclosure [Line Items]
Loans	2,560,495	2,543,778
Consumer | Residential Real Estate | Equity loans
Loans and Leases Receivable Disclosure [Line Items]
Loans	420,362	445,709
Consumer | Consumer | Credit card
Loans and Leases Receivable Disclosure [Line Items]
Loans	570,311	604,881
Consumer | Consumer | Consumer direct
Loans and Leases Receivable Disclosure [Line Items]
Loans	1,329,242	1,254,641
Consumer | Consumer | Consumer indirect
Loans and Leases Receivable Disclosure [Line Items]
Loans	$ 3,078,748	$ 3,134,948