Loans and Leases (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged	$ 6,340,000		$ 6,340,000		$ 6,080,000
Advances from Federal Home Loan Banks	264,700		264,700		835,200
Federal Home Loan Bank Advances, Current Borrowing Capacity	6,080,000		6,080,000		5,240,000
Unamortized Loan Commitment and Origination Fees and Unamortized Discounts or Premiums	200		200		1,700
Loans and Leases Receivable, Gross, Carrying Amount, Covered	58,900		58,900
Federal Reserve Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged	2,200,000		2,200,000		2,080,000
Loans held for sale	46,082		46,082		51,179
Transfer of Portfolio Loans and Leases to Held-for-sale				$ 161,719
Proceeds From Sales of Loans	165,939	$ 307,418	468,705	649,503
Proceeds from sales of portfolio loans			0	162,486
Gain (Loss) on sale of loans and leases			0	1,007
Non-PCI Loans
Financing Receivable, Recorded Investment [Line Items]
Nonaccruing	85,419		85,419		92,534
Accretion Income	2,900	4,200	9,900	10,200
PCI Loans
Financing Receivable, Recorded Investment [Line Items]
Nonaccruing	1,500		1,500		600
Accretion Income	13,500	$ 19,200	45,700	$ 59,000
HomeBancorp Inc.
Financing Receivable, Recorded Investment [Line Items]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Purchase Discount Remaining	6,700		6,700
Guaranty Bank
Financing Receivable, Recorded Investment [Line Items]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Purchase Discount Remaining	11,400		11,400		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	1,500		1,500		2,700
Bancorporation
Financing Receivable, Recorded Investment [Line Items]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Purchase Discount Remaining	13,300		13,300		18,100
Federal Home Loan Bank of Atlanta
Financing Receivable, Recorded Investment [Line Items]
Loans Pledged as Collateral	9,070,000		9,070,000		8,750,000
Federal Reserve Bank
Financing Receivable, Recorded Investment [Line Items]
Loans Pledged as Collateral	$ 2,910,000		$ 2,910,000		$ 2,770,000