Loans Held for Investment (LHFI) and Allowance for Loan Losses, LHFI - Additional Information (Details 1) $ in Millions	9 Months Ended
	Sep. 30, 2021 USD ($) Region	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Accounts, Notes, Loans and Financing Receivable, Gross, Allowance, and Net [Abstract]
Accrued interest receivable	$ 27.9		$ 33.0
Maximum concentration of loan as a percentage of total LHFI	10.00%
Key market regions | Region	5
LHFI classified as TDRs	$ 23.5	$ 25.4
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	20.1	13.3
Unused commitments on TDRs	1.0	4.2
Financing receivable, related allowance	2.6	2.6
Financing receivable, related charge-offs	$ 3.7	$ 2.3
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	$ 20.0		$ 34.2