Loans and Allowance for Credit Losses Credit Quality Indicators (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	$ 13,854,118	$ 13,124,093
Commercial – industrial, financial and agricultural
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	4,088,962	3,725,567
Real estate – commercial mortgage
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	5,462,330	5,197,155
Commercial - secured
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	3,926,113	3,564,614
Commercial - unsecured
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	162,849	160,953
Construction - commercial residential
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	179,303	203,670
Construction - commercial
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	559,991	427,419
Total real estate - construction (excluding construction - other)
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	739,294	631,089
Commercial Loans, Commerical Mortgages, Constructions Loans
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	$ 10,290,586	$ 9,553,811
% of Total	100.00%	100.00%
Pass | Commercial – industrial, financial and agricultural
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	$ 3,853,434	$ 3,480,166
Pass | Real estate – commercial mortgage
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	5,204,263	4,899,016
Pass | Commercial - secured
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	3,696,692	3,333,486
Pass | Commercial - unsecured
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	156,742	146,680
Pass | Construction - commercial residential
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	140,337	136,109
Pass | Construction - commercial
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	552,710	409,631
Pass | Total real estate - construction (excluding construction - other)
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	693,047	545,740
Pass | Commercial Loans, Commerical Mortgages, Constructions Loans
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	$ 9,750,744	$ 8,924,922
% of Total	94.80%	93.40%
Special Mention | Commercial – industrial, financial and agricultural
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	$ 95,472	$ 128,047
Special Mention | Real estate – commercial mortgage
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	102,625	127,302
Special Mention | Commercial - secured
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	92,711	120,584
Special Mention | Commercial - unsecured
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	2,761	7,463
Special Mention | Construction - commercial residential
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	17,154	27,495
Special Mention | Construction - commercial
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	3,684	12,202
Special Mention | Total real estate - construction (excluding construction - other)
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	20,838	39,697
Special Mention | Commercial Loans, Commerical Mortgages, Constructions Loans
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	$ 218,935	$ 295,046
% of Total	2.10%	3.10%
Substandard or Lower | Commercial – industrial, financial and agricultural
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	$ 140,056	$ 117,354
Substandard or Lower | Real estate – commercial mortgage
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	155,442	170,837
Substandard or Lower | Commercial - secured
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	136,710	110,544
Substandard or Lower | Commercial - unsecured
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	3,346	6,810
Substandard or Lower | Construction - commercial residential
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	21,812	40,066
Substandard or Lower | Construction - commercial
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	3,597	5,586
Substandard or Lower | Total real estate - construction (excluding construction - other)
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	25,409	45,652
Substandard or Lower | Commercial Loans, Commerical Mortgages, Constructions Loans
Financing Receivable, Recorded Investment [Line Items]
Carrying Amount	$ 320,907	$ 333,843
% of Total	3.10%	3.50%