Loans Held for Investment (LHFI) and Allowance for Loan Losses, LHFI - Additional Information (Details 1)	9 Months Ended
	Sep. 30, 2018 USD ($) Region	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Financing Receivable Impaired [Line Items]
Maximum concentration of loan as a percentage of total LHFI	10.00%
Key market regions | Region	5
Carrying amounts of nonaccrual LHFI considered for impairment analysis	$ 67,800,000		$ 67,600,000
Nonaccrual	67,836,000		67,575,000
Minimum loan amount for loan to be specifically reviewed for impairment and deemed impaired	500,000
Amount held by company of foreclosure or in substance repossession of consumer mortgage LHFI classified as TDRs	203,000		366,000
TDRs Classified as Consumer mortgage LHFI in the process of formal disclosure	55,000		0
LHFI Classified as TDRs	36,000,000	$ 24,400,000
LHFI classified as TDRs from credits with interest only payments	24,700,000	21,100,000
Unused Commitments on Troubled Debt Restructurings	7,700,000	0
Financing receivable, related allowance	14,300,000	393,000
Financing receivable, related charge-offs	4,700,000	126,000
Troubled Debt Restructurings [Member]
Financing Receivable Impaired [Line Items]
Nonaccrual	$ 33,617,000	$ 23,939,000	$ 23,200,000