Loans and Allowance for Credit Losses Credit Quality Indicators (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	$ 15,511,209	$ 14,718,662
Real-estate - commercial mortgage
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	6,275,140	6,018,582
Real-estate - commercial mortgage | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	6,028,523	5,763,122
Real-estate - commercial mortgage | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	118,947	132,484
Real-estate - commercial mortgage | December 31, 2016
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	127,670	122,976
Commercial - secured
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	4,088,958	3,933,552
Commercial - secured | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	3,807,138	3,686,152
Commercial - secured | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	98,639	128,873
Commercial - secured | December 31, 2016
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	183,181	118,527
Commercial - unsecured
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	134,117	153,934
Commercial - unsecured | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	127,561	145,922
Commercial - unsecured | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	3,474	4,481
Commercial - unsecured | December 31, 2016
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	3,082	3,531
Commercial - industrial, financial and agricultural
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	4,223,075	4,087,486
Commercial - industrial, financial and agricultural | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	3,934,699	3,832,074
Commercial - industrial, financial and agricultural | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	102,113	133,354
Commercial - industrial, financial and agricultural | December 31, 2016
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	186,263	122,058
Construction - commercial residential
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	156,127	142,189
Construction - commercial residential | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	134,786	113,570
Construction - commercial residential | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	6,746	15,447
Construction - commercial residential | December 31, 2016
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	14,595	13,172
Construction - commercial
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	751,398	644,490
Construction - commercial | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	743,111	635,963
Construction - commercial | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	4,418	3,412
Construction - commercial | December 31, 2016
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	3,869	5,115
Construction - Commercial and Commercial Residential
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	907,525	786,679
Construction - Commercial and Commercial Residential | Pass
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	877,897	749,533
Construction - Commercial and Commercial Residential | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	11,164	18,859
Construction - Commercial and Commercial Residential | December 31, 2016
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	18,464	18,287
Commercial Loans, Commerical Mortgages, Constructions Loans
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	$ 11,405,740	$ 10,892,747
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	$ 10,841,119	$ 10,344,729
Percentage of total loans rated	95.10%	95.00%
Commercial Loans, Commerical Mortgages, Constructions Loans | Special Mention
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	$ 232,224	$ 284,697
Percentage of total loans rated	2.00%	2.60%
Commercial Loans, Commerical Mortgages, Constructions Loans | December 31, 2016
Internal risk ratings for loans by segment [Line Items]
Loans and Leases Receivable, Gross	$ 332,397	$ 263,321
Percentage of total loans rated	2.90%	2.40%