Loans and Leases Held for Investment and Credit Quality - Summary of Credit Quality Indicators by Portfolio Class (Parenthetical) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Financing Receivable, Recorded Investment [Line Items]
Total loans and leases receivable	$ 4,349,274	$ 1,797,386
Loans receivable, fair value	815,374	824,520
Loans Insured or Guaranteed by US Government Authorities
Financing Receivable, Recorded Investment [Line Items]
Total loans and leases receivable	2,610,000	622,600
Loans Insured or Guaranteed by US Government Authorities | Risk Grades 1 - 4
Financing Receivable, Recorded Investment [Line Items]
Total loans and leases receivable	2,440,000	556,800
Loans Insured or Guaranteed by US Government Authorities | Risk Grade 5
Financing Receivable, Recorded Investment [Line Items]
Total loans and leases receivable	128,000	42,700
Loans Insured or Guaranteed by US Government Authorities | Risk Grades 6 - 8
Financing Receivable, Recorded Investment [Line Items]
Total loans and leases receivable	$ 40,900	$ 23,100