Loans and Allowance for Credit Losses Allowance for Loan Losses by Portfolio Segment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans and Leases Receivable, Allowance, Beginning Balance	$ 168,679	$ 169,054
Loans charged off	(33,290)	(33,927)	$ (32,157)
Recoveries of loans previously charged off	14,744	20,658	15,388
Loans Charged Off, Net Of Recoveries	(18,546)	(13,269)	(16,769)
Provision for loan losses	19,777	12,894
Financing receivable, allowance for credit losses, collectively evaluated for impairment	128,389	121,562
Financing receivable, allowance for credit losses, individually evaluated for impairment	41,521	47,117
Loans and Leases Receivable, Allowance, Ending Balance	169,910	168,679	169,054
Measured for impairment under FASB ASC Subtopic 450-20	15,577,118	14,519,579
Evaluated for impairment under FASB ASC Section 310-10-35	191,129	179,693
Loans, net of unearned income	15,768,247	14,699,272
Change in provision allocated to commitments to lend to borrowers	3,528	290
Provision for credit losses	23,305	13,182	2,250
Real estate – commercial mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans and Leases Receivable, Allowance, Beginning Balance	46,842	47,866
Loans charged off	(2,169)	(3,580)
Recoveries of loans previously charged off	1,668	3,373
Loans Charged Off, Net Of Recoveries	(501)	(207)
Provision for loan losses	12,452	(817)
Financing receivable, allowance for credit losses, collectively evaluated for impairment	50,681	36,680
Financing receivable, allowance for credit losses, individually evaluated for impairment	8,112	10,162
Loans and Leases Receivable, Allowance, Ending Balance	58,793	46,842	47,866
Measured for impairment under FASB ASC Subtopic 450-20	6,316,023	5,963,689
Evaluated for impairment under FASB ASC Section 310-10-35	48,781	54,893
Loans, net of unearned income	6,364,804	6,018,582
Commercial – industrial, financial and agricultural
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans and Leases Receivable, Allowance, Beginning Balance	54,353	57,098
Loans charged off	(19,067)	(15,276)
Recoveries of loans previously charged off	7,771	8,981
Loans Charged Off, Net Of Recoveries	(11,296)	(6,295)
Provision for loan losses	23,223	3,550
Financing receivable, allowance for credit losses, collectively evaluated for impairment	54,874	40,700
Financing receivable, allowance for credit losses, individually evaluated for impairment	11,406	13,653
Loans and Leases Receivable, Allowance, Ending Balance	66,280	54,353	57,098
Measured for impairment under FASB ASC Subtopic 450-20	4,236,572	4,038,511
Evaluated for impairment under FASB ASC Section 310-10-35	63,725	48,975
Loans, net of unearned income	4,300,297	4,087,486
Real estate – home equity
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans and Leases Receivable, Allowance, Beginning Balance	26,801	22,405
Loans charged off	(2,340)	(4,912)
Recoveries of loans previously charged off	813	1,171
Loans Charged Off, Net Of Recoveries	(1,527)	(3,741)
Provision for loan losses	(7,147)	8,137
Financing receivable, allowance for credit losses, collectively evaluated for impairment	7,003	17,290
Financing receivable, allowance for credit losses, individually evaluated for impairment	11,124	9,511
Loans and Leases Receivable, Allowance, Ending Balance	18,127	26,801	22,405
Measured for impairment under FASB ASC Subtopic 450-20	1,535,026	1,605,910
Evaluated for impairment under FASB ASC Section 310-10-35	24,693	19,205
Loans, net of unearned income	1,559,719	1,625,115
Real estate – residential mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans and Leases Receivable, Allowance, Beginning Balance	22,929	21,375
Loans charged off	(687)	(2,326)
Recoveries of loans previously charged off	786	1,072
Loans Charged Off, Net Of Recoveries	99	(1,254)
Provision for loan losses	(6,940)	2,808
Financing receivable, allowance for credit losses, collectively evaluated for impairment	6,193	11,032
Financing receivable, allowance for credit losses, individually evaluated for impairment	9,895	11,897
Loans and Leases Receivable, Allowance, Ending Balance	16,088	22,929	21,375
Measured for impairment under FASB ASC Subtopic 450-20	1,913,004	1,555,946
Evaluated for impairment under FASB ASC Section 310-10-35	41,707	46,048
Loans, net of unearned income	1,954,711	1,601,994
Real estate – construction
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans and Leases Receivable, Allowance, Beginning Balance	6,455	6,529
Loans charged off	(3,765)	(1,218)
Recoveries of loans previously charged off	1,582	3,924
Loans Charged Off, Net Of Recoveries	(2,183)	2,706
Provision for loan losses	2,348	(2,780)
Financing receivable, allowance for credit losses, collectively evaluated for impairment	5,653	4,587
Financing receivable, allowance for credit losses, individually evaluated for impairment	967	1,868
Loans and Leases Receivable, Allowance, Ending Balance	6,620	6,455	6,529
Measured for impairment under FASB ASC Subtopic 450-20	994,738	833,117
Evaluated for impairment under FASB ASC Section 310-10-35	12,197	10,532
Loans, net of unearned income	1,006,935	843,649
Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans and Leases Receivable, Allowance, Beginning Balance	3,574	2,585
Loans charged off	(2,227)	(2,800)
Recoveries of loans previously charged off	1,156	1,295
Loans Charged Off, Net Of Recoveries	(1,071)	(1,505)
Provision for loan losses	(458)	2,494
Financing receivable, allowance for credit losses, collectively evaluated for impairment	2,028	3,548
Financing receivable, allowance for credit losses, individually evaluated for impairment	17	26
Loans and Leases Receivable, Allowance, Ending Balance	2,045	3,574	2,585
Measured for impairment under FASB ASC Subtopic 450-20	313,757	291,430
Evaluated for impairment under FASB ASC Section 310-10-35	26	40
Loans, net of unearned income	313,783	291,470
Leasing and other and overdrafts
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans and Leases Receivable, Allowance, Beginning Balance	3,192	2,468
Loans charged off	(3,035)	(3,815)
Recoveries of loans previously charged off	968	842
Loans Charged Off, Net Of Recoveries	(2,067)	(2,973)
Provision for loan losses	832	3,697
Financing receivable, allowance for credit losses, collectively evaluated for impairment	1,957	3,192
Financing receivable, allowance for credit losses, individually evaluated for impairment	0	0
Loans and Leases Receivable, Allowance, Ending Balance	1,957	3,192	2,468
Measured for impairment under FASB ASC Subtopic 450-20	267,998	230,976
Evaluated for impairment under FASB ASC Section 310-10-35	0	0
Loans, net of unearned income	267,998	230,976
Unallocated
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans and Leases Receivable, Allowance, Beginning Balance	4,533	8,728
Loans charged off	0	0
Recoveries of loans previously charged off		0
Loans Charged Off, Net Of Recoveries	0	0
Provision for loan losses	(4,533)	(4,195)
Financing receivable, allowance for credit losses, collectively evaluated for impairment	0	4,533
Loans and Leases Receivable, Allowance, Ending Balance	$ 0	$ 4,533	$ 8,728