Disposals (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Jun. 30, 2014	Dec. 31, 2016
Disposals [Abstract]
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal	$ 4.4
Loans in disposal	27.9
Deposits in disposal	$ 130.6
Restructuring and Related Cost, Incurred Cost		$ 0.4

Major classification and carrying amount of total assets sold as part of the disposal group.

No definition available.

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Namespace Prefix:	oksb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Major classification and carrying amount of total liabilities sold as part of the disposal group.

+ References

No definition available.

+ Details

Name:	oksb_LiabilitiesOfDisposalGroup
Namespace Prefix:	oksb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

+ Details

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

- Definition

Amount before tax of gain (loss) recognized on the sale or disposal of a disposal group. Excludes discontinued operations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=51824106&loc=d3e2443-110228

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51719941&loc=d3e2941-110230

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Discloses the amount charged against the accrued restructuring reserves, or earnings if not previously accrued, during the period for the specified type of restructuring cost.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 420
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SAB TOPIC 5.P.4(b))
-URI http://asc.fasb.org/extlink&oid=27011515&loc=d3e140904-122747

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 420
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6394359&loc=d3e17939-110869

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Staff Accounting Bulletin (SAB)
-Number Topic 5
-Section P
-Subsection 3, 4

+ Details

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Balance Type:	debit
Period Type:	duration