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Securities - Contractural Maturities (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended |
--- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013
Securities Available-for-Sale, Amortized Cost |  |
Due in one year or less | $ 8,410 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost |
Due after one year through five years | 5,804 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost |
Due after five years through ten years | 36,773 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCost |
Due after ten years | 5,178 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCost |
Debt securities excluding securities not due at a single maturity date | 56,165 osbc_AvailableForSaleDebtMaturitiesBeforeSecuritiesWithoutSingleMaturityDateAmortizedCostBasis |
Total | 391,694 us-gaap_AvailableForSaleSecuritiesAmortizedCost | 376,286 us-gaap_AvailableForSaleSecuritiesAmortizedCost
Securities Available-for-Sale, Weighted Average Yield |  |
Due in one year or less (as a percent) | 1.65%osbc_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCostWeightedAverageYield |
Due after one year through five years (as a percent) | 2.87%osbc_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCostWeightedAverageYield |
Due after five years through ten years (as a percent) | 2.43%osbc_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCostWeightedAverageYield |
Due after ten years (as a percent) | 3.27%osbc_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCostWeightedAverageYield |
Debt securities (as a percent) | 2.43%osbc_AvailableForSaleDebtMaturitiesBeforeSecuritiesWithoutSingleMaturityDateWeightedAverageYield |
Total (as a percent) | 1.94%osbc_AvailableForSaleSecuritiesWeightedAverageYield |
Securities Available-for-Sale, Fair Value |  |
Due in one year or less | 8,417 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue |
Due after one year through five years | 6,042 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue |
Due after five years through ten years | 36,536 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue |
Due after ten years | 5,159 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue |
Debt securities | 56,154 osbc_AvailableForSaleDebtMaturitiesBeforeSecuritiesWithoutSingleMaturityDateFairValue |
Fair Value | 385,486 us-gaap_AvailableForSaleSecurities | 372,191 us-gaap_AvailableForSaleSecurities
Securities Held-to-Maturity, Amortized Cost |  |
Amortized Cost | 259,670 us-gaap_HeldToMaturitySecurities | 256,571 us-gaap_HeldToMaturitySecurities
Securities Held-to-Maturity, Fair Value |  |
Fair Value | 263,266 us-gaap_HeldToMaturitySecuritiesFairValue | 254,328 us-gaap_HeldToMaturitySecuritiesFairValue
Collateralized mortgage obligations |  |
Securities Available-for-Sale, Amortized Cost |  |
Securities not due at a single maturity date | 65,728 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember |
Total | 65,728 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 66,766 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Securities Available-for-Sale, Weighted Average Yield |  |
Securities not due at a single maturity date, Weighted Average Yield (as a percent) | 1.80%osbc_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateWeightedAverageYield / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember |
Securities Available-for-Sale, Fair Value |  |
Securities not due at a single maturity date | 63,627 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember |
Fair Value | 63,627 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 63,876 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Securities Held-to-Maturity, Amortized Cost |  |
Amortized Cost | 222,545 us-gaap_HeldToMaturitySecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 221,303 us-gaap_HeldToMaturitySecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Securities Held-to-Maturity, Fair Value |  |
Fair Value | 224,111 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 219,088 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Asset-backed securities |  |
Securities Available-for-Sale, Amortized Cost |  |
Securities not due at a single maturity date | 175,565 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |
Total | 175,565 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember | 274,118 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Securities Available-for-Sale, Weighted Average Yield |  |
Securities not due at a single maturity date, Weighted Average Yield (as a percent) | 1.14%osbc_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateWeightedAverageYield / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |
Securities Available-for-Sale, Fair Value |  |
Securities not due at a single maturity date | 173,496 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |
Fair Value | 173,496 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember | 273,203 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Collateralized loan obligations |  |
Securities Available-for-Sale, Amortized Cost |  |
Total | 94,236 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedLoanObligationsMember |
Securities Available-for-Sale, Weighted Average Yield |  |
Securities not due at a single maturity date, Weighted Average Yield (as a percent) | 3.22%osbc_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateWeightedAverageYield / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedLoanObligationsMember |
Securities Available-for-Sale, Fair Value |  |
Fair Value | 92,209 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedLoanObligationsMember |
Securities Held-to-Maturity, Amortized Cost |  |
Amortized Cost | 94,236 us-gaap_HeldToMaturitySecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedLoanObligationsMember |
Securities Held-to-Maturity, Fair Value |  |
Fair Value | 92,209 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedLoanObligationsMember |
Mortgage-backed and collateralized mortgage obligations |  |
Securities Available-for-Sale, Weighted Average Yield |  |
Securities not due at a single maturity date, Weighted Average Yield (as a percent) | 3.06%osbc_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateWeightedAverageYield / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = osbc_MortgageBackedAndCollateralizedMortgageObligationsMember |
Securities Held-to-Maturity, Amortized Cost |  |
Amortized Cost | 259,670 us-gaap_HeldToMaturitySecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = osbc_MortgageBackedAndCollateralizedMortgageObligationsMember |
Securities Held-to-Maturity, Fair Value |  |
Fair Value | $ 263,266 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = osbc_MortgageBackedAndCollateralizedMortgageObligationsMember |

X
- Definition Amortized cost of debt securities categorized neither as held-to-maturity nor trading. This item represents the total amortized cost of all debt securities grouped by maturity dates, excluding the securities with no single maturity date. + References No definition available. + Details
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- Definition Fair value of debt securities categorized neither as held-to-maturity nor trading. This item represents the total fair value of all debt securities grouped by maturity dates, excluding the securities with no single maturity date. + References No definition available. + Details
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- Definition Represents the weighted average yield of available-for-sale debt securities maturing after the tenth rolling twelve months following the latest balance sheet presented. This item represents the weighted average yield of all debt securities grouped by maturity dates, excluding the securities with no single maturity date. + References No definition available. + Details
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- Definition Represents the weighted average yield of available-for-sale debt securities maturing in the sixth fiscal year through the tenth fiscal year following the latest fiscal year. + References No definition available. + Details
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- Definition Represents the weighted average yield of available-for-sale debt securities maturing in the second fiscal year through the fifth fiscal year following the latest fiscal year. + References No definition available. + Details
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- Definition Represents the weighted average yield of available-for-sale debt securities maturing after the tenth fiscal year following the latest fiscal year. + References No definition available. + Details
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- Definition Represents the weighted average yield of available-for-sale debt securities maturing in the next fiscal year following the latest fiscal year. + References No definition available. + Details
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- Definition Represents the weighted average yield of available-for-sale debt securities that do not have a single maturity date and which the entity has decided to disclose separately rather than allocating the cost over several maturity groupings. + References No definition available. + Details
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- Definition This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Amount after adjustments of available-for-sale debt securities at cost, maturing in the sixth fiscal year through the tenth fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27198-111563 + Details
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- Definition Fair value of available-for-sale debt securities maturing in the sixth fiscal year through the tenth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27198-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6957658&loc=d3e62586-112803 + Details
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- Definition Amount after adjustments of available-for-sale debt securities at cost, maturing in the second fiscal year through the fifth fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27198-111563 + Details
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- Definition Fair value of available-for-sale debt securities maturing in the second fiscal year through the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27198-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6957658&loc=d3e62586-112803 + Details
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- Definition Amount after adjustments of available-for-sale debt securities at cost, maturing after the tenth fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27198-111563 + Details
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- Definition Amount after adjustments of available-for-sale debt securities at cost, maturing in the next fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27198-111563 + Details
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- Definition Amount of available-for-sale debt securities at cost, net of adjustments, that do not have a single maturity date and which the entity has decided to disclose separately rather than allocating the cost over several maturity groupings. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings (less any cumulative-effect adjustments, as defined) and fair value hedge accounting adjustments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27198-111563 + Details
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