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Acquisition of a Business - Summary of Acquired Non-Impaired Loans as of Acquisition Date (Details) - USD ($) $ in Thousands | Apr. 30, 2019 | May 31, 2018
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Oak Park River Forest Bankshares, Inc. |  |  |
Business Acquisition [Line Items] |  |  |
Fair value |  | $ 204,496 |
Gross contractual amounts receivable |  | 254,755 |
Estimate of contractual cash flows not expected to be collected | [1] | 12,987 |
Estimate of contractual cash flows expected to be collected |  | $ 241,768 |
First Evanston Bancorp, Inc. |  |  |
Business Acquisition [Line Items] |  |  |
Fair value |  |  | $ 890,986
Gross contractual amounts receivable |  |  | 1,057,374
Estimate of contractual cash flows not expected to be collected | [1] |  | 36,544
Estimate of contractual cash flows expected to be collected |  |  | $ 1,020,830
[1] | Includes interest payments not expected to be collected due to loan prepayments as well as principal and interest payments not expected to be collected due to customer default.

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- Definition For receivables acquired in a business combination, excluding certain loans and debt securities acquired in a transfer (as defined), this element represents the best estimate at the acquisition date of the contractual cash flows not expected to be collected, by major class of receivable, such as loans, direct finance leases (as defined), and any other class of receivables. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=116869197&loc=d3e4845-128472 + Details
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