Loans and Allowance for Loan Losses - Credit quality indicators (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Commercial, Financial and Agricultural
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	$ 24,660,668	$ 25,122,002
Commercial Real Estate | Real estate – construction
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	2,227,405	2,125,316
Commercial Real Estate | Commercial real estate – mortgage
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	11,398,821	11,210,660
Residential Real Estate | Residential real estate – mortgage
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	13,117,776	13,259,994
Residential Real Estate | Equity lines of credit
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	2,560,495	2,543,778
Residential Real Estate | Equity loans
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	420,362	445,709
Consumer | Credit card
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	570,311	604,881
Consumer | Consumer direct
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	1,329,242	1,254,641
Consumer | Consumer indirect
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	3,078,748	3,134,948
Commercial
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	38,286,894	38,457,978
Commercial | Commercial, Financial and Agricultural
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	24,660,668	25,122,002
Commercial | Commercial, Financial and Agricultural | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	22,819,424	23,142,975
Commercial | Commercial, Financial and Agricultural | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	713,760	758,417
Commercial | Commercial, Financial and Agricultural | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	957,263	1,081,439
Commercial | Commercial, Financial and Agricultural | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	170,221	139,171
Commercial | Commercial Real Estate | Real estate – construction
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	2,227,405	2,125,316
Commercial | Commercial Real Estate | Commercial real estate – mortgage
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	11,398,821	11,210,660
Commercial | Commercial Real Estate | Pass | Real estate – construction
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	2,181,799	2,055,483
Commercial | Commercial Real Estate | Pass | Commercial real estate – mortgage
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	11,107,996	10,898,877
Commercial | Commercial Real Estate | Special Mention | Real estate – construction
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	41,654	60,826
Commercial | Commercial Real Estate | Special Mention | Commercial real estate – mortgage
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	148,639	187,182
Commercial | Commercial Real Estate | Substandard | Real estate – construction
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	3,889	9,007
Commercial | Commercial Real Estate | Substandard | Commercial real estate – mortgage
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	121,427	106,183
Commercial | Commercial Real Estate | Doubtful | Real estate – construction
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	63	0
Commercial | Commercial Real Estate | Doubtful | Commercial real estate – mortgage
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	20,759	18,418
Consumer
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	21,076,934	21,243,951
Consumer | Residential Real Estate | Residential real estate – mortgage
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	13,117,776	13,259,994
Consumer | Residential Real Estate | Equity lines of credit
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	2,560,495	2,543,778
Consumer | Residential Real Estate | Equity loans
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	420,362	445,709
Consumer | Residential Real Estate | Performing | Residential real estate – mortgage
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	12,973,925	13,115,936
Consumer | Residential Real Estate | Performing | Equity lines of credit
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	2,526,342	2,507,375
Consumer | Residential Real Estate | Performing | Equity loans
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	407,031	431,417
Consumer | Residential Real Estate | Nonperforming | Residential real estate – mortgage
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	143,851	144,058
Consumer | Residential Real Estate | Nonperforming | Equity lines of credit
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	34,153	36,403
Consumer | Residential Real Estate | Nonperforming | Equity loans
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	13,331	14,292
Consumer | Consumer | Credit card
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	570,311	604,881
Consumer | Consumer | Consumer direct
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	1,329,242	1,254,641
Consumer | Consumer | Consumer indirect
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	3,078,748	3,134,948
Consumer | Consumer | Performing | Credit card
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	557,858	593,927
Consumer | Consumer | Performing | Consumer direct
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	1,323,731	1,249,370
Consumer | Consumer | Performing | Consumer indirect
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	3,063,330	3,121,825
Consumer | Consumer | Nonperforming | Credit card
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	12,453	10,954
Consumer | Consumer | Nonperforming | Consumer direct
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	5,511	5,271
Consumer | Consumer | Nonperforming | Consumer indirect
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	$ 15,418	$ 13,123