Loans and Allowance for Credit Losses Credit Quality Indicators (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 13,244,230	$ 13,111,716	$ 12,839,511
Commercial Loans, Commerical Mortgages, Constructions Loans [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 9,719,863	$ 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	$ 5,237,800	$ 5,197,155	5,128,734
Commercial - industrial, financial, and agricultural [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	3,806,699	3,725,567	$ 3,601,721
Commercial - Secured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	3,653,660	3,564,614
Commercial - Unsecured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	153,039	160,953
Construction, Excluding Contstruction Other [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	675,364	631,089
Construction - Commercial Residential [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	186,948	203,670
Construction - Commercial [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	488,416	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	$ 9,112,884	$ 8,924,922
Percentage of total loans rated	93.80%	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	$ 4,943,773	$ 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	3,560,762	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	3,419,331	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	141,431	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	608,349	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	138,834	136,109
Pass [Member] | Construction - Commercial [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	469,515	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	$ 272,555	$ 295,046
Percentage of total loans rated	2.80%	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	$ 114,385	$ 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	127,330	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	123,663	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	3,667	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	30,840	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	17,526	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	13,314	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	$ 334,424	$ 333,843
Percentage of total loans rated	3.40%	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	$ 179,642	$ 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	118,607	117,354
Substandard [Member] | Commercial - Secured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	110,666	110,544
Substandard [Member] | Commercial - Unsecured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	7,941	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	36,175	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	30,588	40,066
Substandard [Member] | Construction - Commercial [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 5,587	$ 5,586