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Summary of Significant Accounting Policies - Securities, Receivables, and Servicing Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
90-Days or Greater Past Due Loans
Period from due date after which accrual of interest income is discontinued	90 days
Allowance for Loan Losses
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method	$ 1.3
Transfers and Servicing of Financial Assets
Mortgage loans serviced for others	$ 638.2	$ 604.2

X

- Definition

Fair value as of the balance sheet date of mortgage loans representing net future revenues from contractually specified servicing fees, late charges, and other ancillary revenues, in excess of future costs related to servicing arrangements.

+ References

No definition available.

+ Details

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Balance Type:	debit
Period Type:	instant

X

- Definition

Represents the period from due date after which accrual of interest income is discontinued when full loan repayment is in doubt or when a loan becomes contractually past due.

+ References

No definition available.

+ Details

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Namespace Prefix:	osbc_
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X

- References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The effect of a change in method or methods for calculating the allowance for credit losses on the current period provision.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524

+ Details

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Period Type:	duration

X

- References

No definition available.

+ Details

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- References

No definition available.

+ Details

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