Loans and Allowance for Credit Losses Credit Quality Indicators (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 13,536,361	$ 13,111,716	$ 13,030,405
Commercial Loans, Commerical Mortgages, Constructions Loans [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 9,982,223	$ 9,553,811
Percentage of total loans rated	100.00%	100.00%
Real-estate commercial mortage [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 5,339,928	$ 5,197,155	5,156,979
Commercial - industrial, financial, and agricultural [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	3,929,908	3,725,567	$ 3,691,262
Commercial - Secured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	3,782,826	3,564,614
Commercial - Unsecured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	147,082	160,953
Construction, Excluding Contstruction Other [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	712,387	631,089
Construction - Commercial Residential [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	190,521	203,670
Construction - Commercial [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	521,866	427,419
Pass [Member] | Commercial Loans, Commerical Mortgages, Constructions Loans [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 9,406,051	$ 8,924,922
Percentage of total loans rated	94.20%	93.40%
Pass [Member] | Real-estate commercial mortage [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 5,028,655	$ 4,899,016
Pass [Member] | Commercial - industrial, financial, and agricultural [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	3,718,098	3,480,166
Pass [Member] | Commercial - Secured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	3,579,389	3,333,486
Pass [Member] | Commercial - Unsecured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	138,709	146,680
Pass [Member] | Construction, Excluding Contstruction Other [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	659,298	545,740
Pass [Member] | Construction - Commercial Residential [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	144,329	136,109
Pass [Member] | Construction - Commercial [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	514,969	409,631
Special Mention [Member] | Commercial Loans, Commerical Mortgages, Constructions Loans [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 252,464	$ 295,046
Percentage of total loans rated	2.50%	3.10%
Special Mention [Member] | Real-estate commercial mortage [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 132,823	$ 127,302
Special Mention [Member] | Commercial - industrial, financial, and agricultural [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	101,185	128,047
Special Mention [Member] | Commercial - Secured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	97,617	120,584
Special Mention [Member] | Commercial - Unsecured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	3,568	7,463
Special Mention [Member] | Construction, Excluding Contstruction Other [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	18,456	39,697
Special Mention [Member] | Construction - Commercial Residential [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	16,763	27,495
Special Mention [Member] | Construction - Commercial [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	1,693	12,202
Substandard [Member] | Commercial Loans, Commerical Mortgages, Constructions Loans [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 323,708	$ 333,843
Percentage of total loans rated	3.30%	3.50%
Substandard [Member] | Real-estate commercial mortage [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 178,450	$ 170,837
Substandard [Member] | Commercial - industrial, financial, and agricultural [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	110,625	117,354
Substandard [Member] | Commercial - Secured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	105,820	110,544
Substandard [Member] | Commercial - Unsecured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	4,805	6,810
Substandard [Member] | Construction, Excluding Contstruction Other [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	34,633	45,652
Substandard [Member] | Construction - Commercial Residential [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	29,429	40,066
Substandard [Member] | Construction - Commercial [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 5,204	$ 5,586