Loans and Allowance for Credit Losses Credit Quality Indicators (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 14,155,159	$ 13,838,602	$ 13,244,230
Commercial Loans, Commerical Mortgages, Constructions Loans [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 10,522,304	$ 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	$ 5,635,347	$ 5,462,330	5,237,800
Commercial - industrial, financial, and agricultural [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	4,099,177	4,088,962	$ 3,806,699
Commercial - Secured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	3,943,741	3,926,113
Commercial - unsecured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	155,436	162,849
Construction, Excluding Construction Other [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	787,780	739,294
Construction - Commercial Residential [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	183,667	179,303
Construction - Commercial [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	604,113	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	$ 9,987,933	$ 9,750,744
Percentage of total loans rated	94.90%	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	$ 5,371,366	$ 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	3,867,779	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	3,718,231	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	149,548	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	748,788	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	151,817	140,337
Pass [Member] | Construction - Commercial [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	596,971	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	$ 258,774	$ 218,935
Percentage of total loans rated	2.50%	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	$ 141,417	$ 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	97,797	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	95,330	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	2,467	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	19,560	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	17,012	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	2,548	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	$ 275,597	$ 320,907
Percentage of total loans rated	2.60%	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	$ 122,564	$ 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	133,601	140,056
Substandard [Member] | Commercial - Secured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	130,180	136,710
Substandard [Member] | Commercial - unsecured [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	3,421	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	19,432	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	14,838	21,812
Substandard [Member] | Construction - Commercial [Member]
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 4,594	$ 3,597