Loans and Allowance for Credit Losses Credit Quality Indicators (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	$ 15,823,043	$ 15,795,918
Real-estate - commercial mortgage
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	6,304,475	6,364,804
Real-estate - commercial mortgage | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	5,990,838	6,066,396
Real-estate - commercial mortgage | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	164,908	147,604
Real-estate - commercial mortgage | December 31, 2017
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	148,729	150,804
Commercial
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	4,108,587	4,132,440
Commercial | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	3,823,052	3,831,485
Commercial | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	139,345	121,842
Commercial | December 31, 2017
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	146,190	179,113
Commercial - unsecured
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	156,015	167,857
Commercial - unsecured | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	147,914	159,620
Commercial - unsecured | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	3,726	5,478
Commercial - unsecured | December 31, 2017
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	4,375	2,759
Commercial - industrial, financial and agricultural
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	4,264,602	4,300,297
Commercial - industrial, financial and agricultural | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	3,970,966	3,991,105
Commercial - industrial, financial and agricultural | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	143,071	127,320
Commercial - industrial, financial and agricultural | December 31, 2017
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	150,565	181,872
Construction
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	143,066	163,102
Construction | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	128,641	143,759
Construction | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	3,996	5,259
Construction | December 31, 2017
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	10,429	14,084
Construction - commercial
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	768,646	765,816
Construction - commercial | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	762,920	761,218
Construction - commercial | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	344	846
Construction - commercial | December 31, 2017
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	5,382	3,752
Construction - Commercial and Commercial Residential
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	911,712	928,918
Construction - Commercial and Commercial Residential | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	891,561	904,977
Construction - Commercial and Commercial Residential | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	4,340	6,105
Construction - Commercial and Commercial Residential | December 31, 2017
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	15,811	17,836
Commercial Loans, Commerical Mortgages, Constructions Loans
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	$ 11,480,789	$ 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	$ 10,853,365	$ 10,962,478
Percentage of total loans rated	94.50%	94.60%
Commercial Loans, Commerical Mortgages, Constructions Loans | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	$ 312,319	$ 281,029
Percentage of total loans rated	2.70%	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	$ 315,105	$ 350,512
Percentage of total loans rated	2.70%	3.00%