Loans Held for Investment and Allowance for Loan Losses (Narrative) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Proceeds from sale of previously charged-off loans	$ 0	$ 500,000
Allowance for loan losses for non-delinquent loans	69,500,000		$ 85,700,000
Allowance for loan losses for delinquent loans	77,900,000		54,300,000
Unused lines of Credit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Off-balance sheet credit exposure	269,300,000		264,200,000
Reserve for Off-balance Sheet Activities
Accounts, Notes, Loans and Financing Receivable [Line Items]
Related accrual for unfunded loan commitments	5,900,000		$ 5,900,000
Financial Institutions Borrower
Accounts, Notes, Loans and Financing Receivable [Line Items]
Originations of loans held for investment, modified	$ 111,500,000	$ 139,200,000