Loans and Allowance for Credit Losses Credit Quality Indicators (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	$ 15,954,326	$ 15,795,918
Real-estate - commercial mortgage
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	6,337,984	6,364,804
Real-estate - commercial mortgage | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	6,032,828	6,066,396
Real-estate - commercial mortgage | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	170,362	147,604
Real-estate - commercial mortgage | December 31, 2017
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	134,794	150,804
Commercial
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	4,128,197	4,132,440
Commercial | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	3,839,283	3,831,485
Commercial | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	150,310	121,842
Commercial | December 31, 2017
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	138,604	179,113
Commercial - unsecured
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	160,626	167,857
Commercial - unsecured | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	152,796	159,620
Commercial - unsecured | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	3,898	5,478
Commercial - unsecured | December 31, 2017
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	3,932	2,759
Commercial - industrial, financial and agricultural
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	4,288,823	4,300,297
Commercial - industrial, financial and agricultural | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	3,992,079	3,991,105
Commercial - industrial, financial and agricultural | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	154,208	127,320
Commercial - industrial, financial and agricultural | December 31, 2017
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	142,536	181,872
Construction
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	136,305	163,102
Construction | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	120,249	143,759
Construction | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	7,677	5,259
Construction | December 31, 2017
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	8,379	14,084
Construction - commercial
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	760,645	765,816
Construction - commercial | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	756,477	761,218
Construction - commercial | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	552	846
Construction - commercial | December 31, 2017
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	3,616	3,752
Construction - Commercial and Commercial Residential
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	896,950	928,918
Construction - Commercial and Commercial Residential | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	876,726	904,977
Construction - Commercial and Commercial Residential | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	8,229	6,105
Construction - Commercial and Commercial Residential | December 31, 2017
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	11,995	17,836
Commercial Loans, Commerical Mortgages, Constructions Loans
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	$ 11,523,757	$ 11,594,019
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	$ 10,901,633	$ 10,962,478
Percentage of total loans rated	94.60%	94.60%
Commercial Loans, Commerical Mortgages, Constructions Loans | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	$ 332,799	$ 281,029
Percentage of total loans rated	2.90%	2.40%
Commercial Loans, Commerical Mortgages, Constructions Loans | December 31, 2017
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	$ 289,325	$ 350,512
Percentage of total loans rated	2.50%	3.00%