Loans and Allowance for Credit Losses Credit Quality Indicators (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	$ 15,724,738	$ 15,795,918
Real-estate - commercial mortgage
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	6,332,508	6,364,804
Real-estate - commercial mortgage | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	6,027,210	6,066,396
Real-estate - commercial mortgage | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	144,809	147,604
Real-estate - commercial mortgage | December 31, 2017
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	160,489	150,804
Commercial
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	4,154,213	4,132,440
Commercial | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	3,873,737	3,831,485
Commercial | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	100,242	121,842
Commercial | December 31, 2017
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	180,234	179,113
Commercial - unsecured
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	144,859	167,857
Commercial - unsecured | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	139,139	159,620
Commercial - unsecured | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	3,378	5,478
Commercial - unsecured | December 31, 2017
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	2,342	2,759
Commercial - industrial, financial and agricultural
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	4,299,072	4,300,297
Commercial - industrial, financial and agricultural | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	4,012,876	3,991,105
Commercial - industrial, financial and agricultural | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	103,620	127,320
Commercial - industrial, financial and agricultural | December 31, 2017
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	182,576	181,872
Construction
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	158,482	163,102
Construction | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	141,587	143,759
Construction | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	4,613	5,259
Construction | December 31, 2017
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	12,282	14,084
Construction - commercial
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	748,796	765,816
Construction - commercial | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	744,274	761,218
Construction - commercial | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	834	846
Construction - commercial | December 31, 2017
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	3,688	3,752
Construction - Commercial and Commercial Residential
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	907,278	928,918
Construction - Commercial and Commercial Residential | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	885,861	904,977
Construction - Commercial and Commercial Residential | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	5,447	6,105
Construction - Commercial and Commercial Residential | December 31, 2017
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	15,970	17,836
Commercial Loans, Commerical Mortgages, Constructions Loans
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	$ 11,538,858	$ 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	$ 10,925,947	$ 10,962,478
Percentage of total loans rated	94.70%	94.60%
Commercial Loans, Commerical Mortgages, Constructions Loans | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	$ 253,876	$ 281,029
Percentage of total loans rated	2.20%	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	$ 359,035	$ 350,512
Percentage of total loans rated	3.10%	3.00%