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Regulatory & Capital Matters - Dividend Restrictions and Deferrals (Details) | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Dividend Restrictions and Deferrals |  |  |  |
Number of previous years retained profit considered for dividend payment |  |  |  | 2 years
Minimum capital requirements, minimum percentage required, 2016 |  |  |  | 0.625%
Minimum capital requirements, minimum percentage required, 2017 |  |  | 1.25% |
Minimum capital requirements, minimum percentage required, 2018 |  | 1.875% |  |
Minimum capital requirements, minimum percentage required, 2019 and thereafter | 2.50% |  |  |

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- References No definition available. + Details
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- Definition Represents the number of previous years retained profits combined with the current years profits used to determine the amount of dividends that may be paid in any calendar year. + References No definition available. + Details
Name: | osbc_DividendRestrictionsNumberOfPreviousYearsRetainedProfit
Namespace Prefix: | osbc_
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- Definition The minimum percentage of Tier 1 Common Capital required to be maintained in the next fiscal year following the latest fiscal year to prevent capital distribution limitations under regulatory capital rules. + References No definition available. + Details
Name: | osbc_TierOneCommonEquityCapitalDistributionLimitationMinimumRegulatoryCapitalPercentageNextTwelveMonths
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Data Type: | num:percentItemType
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- Definition The minimum percentage of Tier 1 Common Capital required to be maintained in year four and thereafter following the latest fiscal year to prevent capital distribution limitations under regulatory capital rules. + References No definition available. + Details
Name: | osbc_TierOneCommonEquityCapitalDistributionLimitationMinimumRegulatoryCapitalPercentageYearFourAndThereafter
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Data Type: | num:percentItemType
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- Definition The minimum percentage of Tier 1 Common Capital required to be maintained in year three following the latest fiscal year to prevent capital distribution limitations under regulatory capital rules. + References No definition available. + Details
Name: | osbc_TierOneCommonEquityCapitalDistributionLimitationMinimumRegulatoryCapitalPercentageYearThree
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Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition The minimum percentage of Tier 1 Common Capital required to be maintained in year two following the latest fiscal year to prevent capital distribution limitations under regulatory capital rules. + References No definition available. + Details
Name: | osbc_TierOneCommonEquityCapitalDistributionLimitationMinimumRegulatoryCapitalPercentageYearTwo
Namespace Prefix: | osbc_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration