Loans and Leases (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Modifications, Recorded Investment	$ 156,130	$ 164,561
Federal Reserve Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged	2,190,000
Loans held for sale	45,505	51,179
Proceeds from sale of all loans	618,140	823,457
Proceeds from sale of portfolio loans	9,591	162,649	$ 77,665
Proceeds from sale of loans held for sale	608,549	660,808	797,123
Loans and Leases Receivable, Collateral for Secured Borrowings	6,360,000	6,080,000
Advances from Federal Home Loan Banks	175,200	835,200
Federal Home Loan Bank Advances, Current Borrowing Capacity	6,180,000	5,240,000
Noncovered loans pledged to secure debt obligations	9,120,000	8,750,000
Loans and Leases Receivable, Gross, Carrying Amount, Covered	55,600
Gain (Loss) on Sale of Loans and Leases	0	1,007	$ 3,758
Net deferred fees on non-PCI loans	100	1,700
Non-PCI Loans
Financing Receivable, Recorded Investment [Line Items]
Accretion Income	12,800	13,600
PCI Loans
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable, Modifications, Recorded Investment	18,220	18,435
Nonaccruing	1,300	624
Loans on the Cost Recovery Method | PCI Loans
Financing Receivable, Recorded Investment [Line Items]
Carrying value of acquired impaired loans	3,300	1,100
Capital Commerce Bancorp
Financing Receivable, Recorded Investment [Line Items]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Purchase Discount Remaining	3,100
HomeBancorp Inc.
Financing Receivable, Recorded Investment [Line Items]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Purchase Discount Remaining	6,300
Guaranty Bank
Financing Receivable, Recorded Investment [Line Items]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Purchase Discount Remaining	10,800	14,200
Cordia Bancorp Inc.
Financing Receivable, Recorded Investment [Line Items]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Purchase Discount Remaining	1,400	2,700
Bancorporation
Financing Receivable, Recorded Investment [Line Items]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Purchase Discount Remaining	12,000	18,100
Medical and Dental
Financing Receivable, Recorded Investment [Line Items]
Loans in the medical, dental or related field	$ 4,980,000	$ 4,860,000
Medical, dental or related fields percentage of total loans and leases	19.50%	20.60%
Federal Reserve Bank
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Collateral for Secured Borrowings		$ 2,080,000
Loans Pledged as Collateral	$ 2,940,000	$ 2,770,000