Summary of Significant Accounting Policies - Allowance for Credit Losses (Details) - USD ($)	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
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	94,500,000
Liability Recorded for Expected Credit Losses on Unfunded Commitments	$ 21,100,000
SSB Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Threshold Net Book Balance, Individually Evaluated Loans		$ 250,000
CSFL Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Baseline Forecast Probability Of The United States Economy Performing Better Or Worse	50.00%
Threshold Net Book Balance, Individually Evaluated Loans		$ 500,000
Other Assets
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Interest Receivable	$ 94,500,000
Other Liabilities
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Liability Recorded for Expected Credit Losses on Unfunded Commitments	21,100,000
Commercial loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Eligible for the expedited Microbusiness process	$ 100,000
Commercial loans | SSB Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Baseline Forecast Probability Of The United States Economy Performing Better Or Worse	50.00%
Reasonable and supportable forecast period	2 years
Commercial loans | Minimum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Eligible for the expedited BEX process	$ 100,000
Commercial loans | Maximum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Eligible for the expedited BEX process	$ 1,500,000