Summary of Significant Accounting Policies - Allowance for Credit Losses (Details)	Sep. 30, 2020 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Held-to-maturity Securities	$ 0
Accrued interest receivable for investment securities	11,400,000
Threshold Net Book Balance, Individually Evaluated Loans	1,000,000.0
Interest Receivable	97,800,000
Liability Recorded for Expected Credit Losses on Unfunded Commitments	43,200,000
Other Assets
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Interest Receivable	97,800,000
Other Liabilities
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Liability Recorded for Expected Credit Losses on Unfunded Commitments	$ 43,200,000