Loans Held for Investment (LHFI) and Allowance for Loan Losses, LHFI - Additional Information (Details 1)	3 Months Ended
	Mar. 31, 2021 USD ($) Region	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)
Accounts Notes Loans And Financing Receivable Gross Allowance And Net [Abstract]
Accrued interest receivable	$ 32,800,000		$ 33,000,000.0
Maximum concentration of loan as a percentage of total LHFI	10.00%
Key market regions | Region	5
LHFI classified as TDRs	$ 22,900,000	$ 26,500,000
LHFI classified as TDRs from credits with interest only payments and payment concessions	17,300,000
LHFI classified as TDRs from credits with interest only payments and credits renewed at a rate that was not commensurate with that of new debt with similar risk		18,300,000
Unused commitments on TDRs	0	10,700,000
Financing receivable, related allowance	7,500,000	210,000
Financing receivable, related charge-offs	$ 0	$ 2,200,000
Period of extension on interest part of loans modified related to COVID-19	90 days
Period of extension on full payment deferrals part of loans modified related to COVID-19	90 days
Balance of loans remaining under some type of concession related to COVID-19	$ 28,100,000		$ 34,200,000