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Series B Preferred Stock - Dividends and Redemptions (Details) (USD $)
0 Months Ended 1 Months Ended 3 Months Ended
Feb. 15, 2015
Dec. 30, 2014
May 15, 2014
Apr. 28, 2014
May 31, 2014
Mar. 31, 2015
item
Jun. 30, 2014
Dec. 31, 2014
Dec. 30, 2014
Dec. 31, 2013
Preferred Stock                    
Payments of Dividends           $ 1,006,000us-gaap_PaymentsOfDividendsPreferredStockAndPreferenceStock        
Preferred stock           31,553,000us-gaap_PreferredStockValue   47,331,000us-gaap_PreferredStockValue    
Preferred Stock, Shares Outstanding           31,553us-gaap_PreferredStockSharesOutstanding   47,331us-gaap_PreferredStockSharesOutstanding    
Series B Stock                    
Preferred Stock                    
Number of quarters in which if the Company did not pay dividends, will allow the holder the right to appoint representatives to the Company's board of directors           6osbc_PreferredStockNumberOfQuartersDividendsInArrears
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesBPreferredStockMember
       
Repurchase price percentage of liquidation value (as a percent)       94.75%osbc_PreferredStockRepurchasePricePercentageOfLiquidationValue
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesBPreferredStockMember
    94.75%osbc_PreferredStockRepurchasePricePercentageOfLiquidationValue
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesBPreferredStockMember
     
Payments of Dividends 710,000us-gaap_PaymentsOfDividendsPreferredStockAndPreferenceStock
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesBPreferredStockMember
  10,300,000us-gaap_PaymentsOfDividendsPreferredStockAndPreferenceStock
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesBPreferredStockMember
  10,300,000us-gaap_PaymentsOfDividendsPreferredStockAndPreferenceStock
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesBPreferredStockMember
         
Payments for repurchase of preferred stock       24,300,000us-gaap_PaymentsForRepurchaseOfPreferredStockAndPreferenceStock
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesBPreferredStockMember
    24,300,000us-gaap_PaymentsForRepurchaseOfPreferredStockAndPreferenceStock
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesBPreferredStockMember
     
Shares redeemed (in shares)       25,669us-gaap_StockRedeemedOrCalledDuringPeriodShares
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesBPreferredStockMember
  15,778us-gaap_StockRedeemedOrCalledDuringPeriodShares
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesBPreferredStockMember
25,669us-gaap_StockRedeemedOrCalledDuringPeriodShares
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesBPreferredStockMember
     
Preferred stock           $ 31,600,000us-gaap_PreferredStockValue
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesBPreferredStockMember
  $ 47,300,000us-gaap_PreferredStockValue
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesBPreferredStockMember
  $ 72,900,000us-gaap_PreferredStockValue
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesBPreferredStockMember
Shares to be redeemed, percent of issued and outstanding   33.33%osbc_StockRedeemedOrCalledDuringPeriodPercentageOfIssuedAndOutstandingShares
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesBPreferredStockMember
      33.33%osbc_StockRedeemedOrCalledDuringPeriodPercentageOfIssuedAndOutstandingShares
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesBPreferredStockMember
       
Preferred Stock, Shares Outstanding   47,331us-gaap_PreferredStockSharesOutstanding
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesBPreferredStockMember
      31,553us-gaap_PreferredStockSharesOutstanding
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesBPreferredStockMember
  47,331us-gaap_PreferredStockSharesOutstanding
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesBPreferredStockMember
47,331us-gaap_PreferredStockSharesOutstanding
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesBPreferredStockMember
 
Preferred Stock, Redemption Price Per Share   $ 1,000us-gaap_PreferredStockRedemptionPricePerShare
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesBPreferredStockMember
            $ 1,000us-gaap_PreferredStockRedemptionPricePerShare
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesBPreferredStockMember