Loans and Allowance for Credit Losses Credit Quality Indicators (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	$ 14,718,662	$ 13,854,118
Commercial – industrial, financial and agricultural
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	4,087,486	4,088,962
Real estate – commercial mortgage
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	6,018,582	5,462,330
Commercial - secured
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	3,933,552	3,926,113
Commercial - unsecured
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	153,934	162,849
Construction - commercial residential
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	142,189	179,303
Construction - commercial
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	644,490	559,991
Total real estate - construction (excluding construction - other)
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	786,679	739,294
Commercial Loans, Commerical Mortgages, Constructions Loans
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	$ 10,892,747	$ 10,290,586
% of Total	100.00%	100.00%
Pass | Commercial – industrial, financial and agricultural
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	$ 3,832,074	$ 3,853,434
Pass | Real estate – commercial mortgage
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	5,763,122	5,204,263
Pass | Commercial - secured
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	3,686,152	3,696,692
Pass | Commercial - unsecured
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	145,922	156,742
Pass | Construction - commercial residential
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	113,570	140,337
Pass | Construction - commercial
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	635,963	552,710
Pass | Total real estate - construction (excluding construction - other)
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	749,533	693,047
Pass | Commercial Loans, Commerical Mortgages, Constructions Loans
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	$ 10,344,729	$ 9,750,744
% of Total	95.00%	94.80%
Special Mention | Commercial – industrial, financial and agricultural
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	$ 133,354	$ 95,472
Special Mention | Real estate – commercial mortgage
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	132,484	102,625
Special Mention | Commercial - secured
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	128,873	92,711
Special Mention | Commercial - unsecured
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	4,481	2,761
Special Mention | Construction - commercial residential
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	15,447	17,154
Special Mention | Construction - commercial
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	3,412	3,684
Special Mention | Total real estate - construction (excluding construction - other)
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	18,859	20,838
Special Mention | Commercial Loans, Commerical Mortgages, Constructions Loans
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	$ 284,697	$ 218,935
% of Total	2.60%	2.10%
Substandard or Lower | Commercial – industrial, financial and agricultural
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	$ 122,058	$ 140,056
Substandard or Lower | Real estate – commercial mortgage
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	122,976	155,442
Substandard or Lower | Commercial - secured
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	118,527	136,710
Substandard or Lower | Commercial - unsecured
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	3,531	3,346
Substandard or Lower | Construction - commercial residential
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	13,172	21,812
Substandard or Lower | Construction - commercial
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	5,115	3,597
Substandard or Lower | Total real estate - construction (excluding construction - other)
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	18,287	25,409
Substandard or Lower | Commercial Loans, Commerical Mortgages, Constructions Loans
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	$ 263,321	$ 320,907
% of Total	2.40%	3.10%