Loans and Allowance for Credit Losses Credit Quality Indicators (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	$ 16,713,704	$ 16,192,439
Real-estate - commercial mortgage
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	6,604,634	6,434,285
Real-estate - commercial mortgage | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	6,290,567	6,129,463
Real-estate - commercial mortgage | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	142,136	170,827
Real-estate - commercial mortgage | 43465
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	171,931	133,995
Commercial
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	4,310,032	4,224,980
Commercial | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	3,926,856	3,902,484
Commercial | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	198,592	193,470
Commercial | 43465
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	184,584	129,026
Commercial - unsecured
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	184,464	179,568
Commercial - unsecured | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	175,761	171,589
Commercial - unsecured | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	5,082	4,016
Commercial - unsecured | 43465
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	3,621	3,963
Commercial - industrial, financial and agricultural
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	4,494,496	4,404,548
Commercial - industrial, financial and agricultural | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	4,102,617	4,074,073
Commercial - industrial, financial and agricultural | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	203,674	197,486
Commercial - industrial, financial and agricultural | 43465
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	188,205	132,989
Construction
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	118,661	117,872
Construction | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	112,163	104,079
Construction | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	2,871	6,912
Construction | 43465
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	3,627	6,881
Construction - commercial
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	710,713	726,726
Construction - commercial | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	707,290	723,030
Construction - commercial | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	719	1,163
Construction - commercial | 43465
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	2,704	2,533
Construction - Commercial and Commercial Residential
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	829,374	844,598
Construction - Commercial and Commercial Residential | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	819,453	827,109
Construction - Commercial and Commercial Residential | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	3,590	8,075
Construction - Commercial and Commercial Residential | 43465
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	6,331	9,414
Commercial Loans, Commerical Mortgages, Constructions Loans
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	$ 11,928,504	$ 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	$ 11,212,637	$ 11,030,645
Percentage of total loans rated	94.00%	94.40%
Commercial Loans, Commerical Mortgages, Constructions Loans | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	$ 349,400	$ 376,388
Percentage of total loans rated	2.90%	3.20%
Commercial Loans, Commerical Mortgages, Constructions Loans | 43465
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	$ 366,467	$ 276,398
Percentage of total loans rated	3.10%	2.40%