Nature of Operations and Summary of Significant Accounting Policies - Other (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Cash and Cash Equivalents
Cash accounts in excess of federally insured limits	$ 492,100,000	$ 331,200,000
Cash accounts in excess of federally insured limits with Federal Reserve Bank	478,800,000	295,400,000
Cash accounts in excess of federally insured limits with Federal Home Loan Bank of Indianapolis	2,300,000	12,900,000
Cash accounts in excess of federally insured limits with Federal Home Loan Bank of Chicago	20,000	15,000
Securities
Securities, held-to-maturity	$ 0	$ 0
Loans Held for Sale under Mortgage Banking Activities
Maximum participation interest to be purchased in individual loans (as a percent)	100.00%