Loans and Allowance for Credit Losses Credit Quality Indicators (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	$ 16,395,670	$ 16,192,439
Real-estate - commercial mortgage
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	6,497,973	6,434,285
Real-estate - commercial mortgage | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	6,173,883	6,129,463
Real-estate - commercial mortgage | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	162,425	170,827
Real-estate - commercial mortgage | 43465
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	161,665	133,995
Commercial
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	4,189,596	4,224,980
Commercial | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	3,835,171	3,902,484
Commercial | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	182,569	193,470
Commercial | 43465
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	171,856	129,026
Commercial - unsecured
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	175,652	179,568
Commercial - unsecured | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	168,311	171,589
Commercial - unsecured | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	4,972	4,016
Commercial - unsecured | 43465
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	2,369	3,963
Commercial - industrial, financial and agricultural
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	4,365,248	4,404,548
Commercial - industrial, financial and agricultural | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	4,003,482	4,074,073
Commercial - industrial, financial and agricultural | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	187,541	197,486
Commercial - industrial, financial and agricultural | 43465
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	174,225	132,989
Construction
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	116,209	117,872
Construction | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	109,168	104,079
Construction | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	3,082	6,912
Construction | 43465
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	3,959	6,881
Construction - commercial
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	729,484	726,726
Construction - commercial | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	725,556	723,030
Construction - commercial | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	731	1,163
Construction - commercial | 43465
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	3,197	2,533
Construction - Commercial and Commercial Residential
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	845,693	844,598
Construction - Commercial and Commercial Residential | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	834,724	827,109
Construction - Commercial and Commercial Residential | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	3,813	8,075
Construction - Commercial and Commercial Residential | 43465
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	7,156	9,414
Commercial Loans, Commerical Mortgages, Constructions Loans
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	$ 11,708,914	$ 11,683,431
Percentage of total loans rated	100.00%	100.00%
Commercial Loans, Commerical Mortgages, Constructions Loans | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	$ 11,012,089	$ 11,030,645
Percentage of total loans rated	94.10%	94.40%
Commercial Loans, Commerical Mortgages, Constructions Loans | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	$ 353,779	$ 376,388
Percentage of total loans rated	3.00%	3.20%
Commercial Loans, Commerical Mortgages, Constructions Loans | 43465
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	$ 343,046	$ 276,398
Percentage of total loans rated	2.90%	2.40%