Loans And Allowance For Loan Losses (Recorded Investment And The Number Of Loans Modified As Troubled Debt Restructuring Which Subsequently Defaulted) (Details)	24 Months Ended
Dec. 31, 2016 loan
Loans And Allowance For Loan Losses [Abstract]
Number of Contracts	0

Number of financing receivables that have been modified by troubled debt restructurings within the previous 12 months and for which there was a payment default.

No definition available.

Name:	oksb_FinancingReceivableModificationNumberOfContracts
Namespace Prefix:	oksb_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration