Loans and Leases (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Sep. 30, 2019
Financing Receivable, Recorded Investment [Line Items]
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged	$ 6,570,000	$ 6,360,000
Federal Reserve Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged	2,980,000	2,190,000
Loans held for sale	67,900	45,500
Proceeds from Sale of Loans Receivable	756,000	618,100
Loans and Leases Receivable, Gain (Loss) on Sales, Net	299
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Purchase Discount Remaining	30,900	33,300
Advances from Federal Home Loan Banks		175,200
Federal Home Loan Bank Advances, Current Borrowing Capacity	6,010,000	6,180,000
Net deferred fees on non-PCI loans	927	79
Loans Held For Sale
Financing Receivable, Recorded Investment [Line Items]
Proceeds from Sale of Loans Receivable	731,800	608,500
Portfolio Loans
Financing Receivable, Recorded Investment [Line Items]
Proceeds from Sale of Loans Receivable	24,200	9,600
Non-PCI Loans
Financing Receivable, Recorded Investment [Line Items]
Nonaccruing	114,946	84,546
Accretion Income	13,200	12,800
Loans and leases greater than 90 days and accruing	3,291	2,888
Non-PCI Loans | Commercial
Financing Receivable, Recorded Investment [Line Items]
Nonaccruing	42,407	18,529
Loans and leases greater than 90 days and accruing	1,094	808
PCI Loans
Financing Receivable, Recorded Investment [Line Items]
Nonaccruing		1,300	$ 6,700
PCI Loans | Commercial
Financing Receivable, Recorded Investment [Line Items]
Loans and leases greater than 90 days and accruing	24,300	37,000
Medical and Dental
Financing Receivable, Recorded Investment [Line Items]
Loans in the medical, dental or related field	$ 5,160,000	$ 4,980,000
Medical, dental or related fields percentage of total loans and leases	17.90%	19.50%
Federal Home Loan Bank of Atlanta
Financing Receivable, Recorded Investment [Line Items]
Loans Pledged as Collateral	$ 9,410,000	$ 9,120,000
Advances from Federal Home Loan Banks	563,700
Federal Reserve Bank
Financing Receivable, Recorded Investment [Line Items]
Loans Pledged as Collateral	$ 3,680,000	2,940,000
Loans on the Cost Recovery Method
Financing Receivable, Recorded Investment [Line Items]
Carrying value of acquired impaired loans		$ 3,300	$ 2,900