Loans and Allowance for Credit Losses Credit Quality Indicators (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 13,870,701	$ 13,838,602	$ 13,115,505
Commercial Loans, Commerical Mortgages, Constructions Loans [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 10,342,536	$ 10,290,586
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	$ 5,558,108	$ 5,462,330	5,227,101
Commercial - industrial, financial, and agricultural [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	4,035,333	4,088,962	$ 3,762,631
Commercial - Secured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	3,881,697	3,926,113
Commercial - Unsecured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	153,636	162,849
Construction, Excluding Construction Other [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	749,095	739,294
Construction - Commercial Residential [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	182,279	179,303
Construction - Commercial [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	566,816	559,991
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	$ 9,805,654	$ 9,750,744
Percentage of total loans rated	94.80%	94.80%
Pass [Member] | Real-estate commercial mortage [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 5,283,340	$ 5,204,263
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	3,816,373	3,853,434
Pass [Member] | Commercial - Secured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	3,668,743	3,696,692
Pass [Member] | Commercial - Unsecured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	147,630	156,742
Pass [Member] | Construction, Excluding Construction Other [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	705,941	693,047
Pass [Member] | Construction - Commercial Residential [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	146,590	140,337
Pass [Member] | Construction - Commercial [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	559,351	552,710
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	$ 222,941	$ 218,935
Percentage of total loans rated	2.20%	2.10%
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	$ 121,889	$ 102,625
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	81,142	95,472
Special Mention [Member] | Commercial - Secured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	78,508	92,711
Special Mention [Member] | Commercial - Unsecured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	2,634	2,761
Special Mention [Member] | Construction, Excluding Construction Other [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	19,910	20,838
Special Mention [Member] | Construction - Commercial Residential [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	17,068	17,154
Special Mention [Member] | Construction - Commercial [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	2,842	3,684
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	$ 313,941	$ 320,907
Percentage of total loans rated	3.00%	3.10%
Substandard [Member] | Real-estate commercial mortage [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 152,879	$ 155,442
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	137,818	140,056
Substandard [Member] | Commercial - Secured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	134,446	136,710
Substandard [Member] | Commercial - Unsecured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	3,372	3,346
Substandard [Member] | Construction, Excluding Construction Other [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	23,244	25,409
Substandard [Member] | Construction - Commercial Residential [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	18,621	21,812
Substandard [Member] | Construction - Commercial [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 4,623	$ 3,597