Investment Securities (Summary Of Securities With Gross Unrealized Losses And Their Fair Values) (Details) $ in Thousands	Mar. 31, 2017 USD ($) security	Dec. 31, 2016 USD ($) security
Available-for-sale Securities and Held-to-maturity Securities
Number of Securities, Available for Sale \| security	121	143
Number of Securities, Held to Maturity \| security	5	7
Amortized cost of securities with unrealized losses, Available for Sale	$ 253,622	$ 293,273
Amortized cost of securities with unrealized losses, Held to Maturity	442	1,987
Continuous Unrealized Loss Existing for Less Than 12 Months, Available for Sale	(1,351)	(1,874)
Continuous Unrealized Loss Existing for Less Than 12 Months, Held to Maturity	(2)	(8)
Continuous Unrealized Loss Existing for More Than 12 Months, Available for Sale	(537)	(641)
Fair value of securities with unrealized losses, Available for Sale	251,734	290,758
Fair value of securities with unrealized losses, Held to Maturity	$ 440	$ 1,979
Federal Agency Securities [Member]
Available-for-sale Securities and Held-to-maturity Securities
Number of Securities, Available for Sale \| security	14	13
Amortized cost of securities with unrealized losses, Available for Sale	$ 34,729	$ 34,734
Continuous Unrealized Loss Existing for Less Than 12 Months, Available for Sale	(111)	(111)
Continuous Unrealized Loss Existing for More Than 12 Months, Available for Sale	(79)	(87)
Fair value of securities with unrealized losses, Available for Sale	$ 34,539	$ 34,536
Obligations Of State And Political Subdivisions [Member]
Available-for-sale Securities and Held-to-maturity Securities
Number of Securities, Available for Sale \| security	18	36
Number of Securities, Held to Maturity \| security	5	7
Amortized cost of securities with unrealized losses, Available for Sale	$ 9,739	$ 18,283
Amortized cost of securities with unrealized losses, Held to Maturity	442	1,987
Continuous Unrealized Loss Existing for Less Than 12 Months, Available for Sale	(57)	(145)
Continuous Unrealized Loss Existing for Less Than 12 Months, Held to Maturity	(2)	(8)
Continuous Unrealized Loss Existing for More Than 12 Months, Available for Sale	(48)	(46)
Fair value of securities with unrealized losses, Available for Sale	9,634	18,092
Fair value of securities with unrealized losses, Held to Maturity	$ 440	$ 1,979
Residential Mortgage-Backed Securities [Member]
Available-for-sale Securities and Held-to-maturity Securities
Number of Securities, Available for Sale \| security	86	89
Amortized cost of securities with unrealized losses, Available for Sale	$ 200,195	$ 225,986
Continuous Unrealized Loss Existing for Less Than 12 Months, Available for Sale	(1,183)	(1,618)
Continuous Unrealized Loss Existing for More Than 12 Months, Available for Sale	(315)	(348)
Fair value of securities with unrealized losses, Available for Sale	$ 198,697	$ 224,020
Asset-Backed Securities [Member]
Available-for-sale Securities and Held-to-maturity Securities
Number of Securities, Available for Sale \| security	1	3
Amortized cost of securities with unrealized losses, Available for Sale	$ 3,952	$ 9,265
Continuous Unrealized Loss Existing for More Than 12 Months, Available for Sale	(34)	(88)
Fair value of securities with unrealized losses, Available for Sale	$ 3,918	$ 9,177
Corporate Debt [Member]
Available-for-sale Securities and Held-to-maturity Securities
Number of Securities, Available for Sale \| security	2	2
Amortized cost of securities with unrealized losses, Available for Sale	$ 5,007	$ 5,005
Continuous Unrealized Loss Existing for More Than 12 Months, Available for Sale	(61)	(72)
Fair value of securities with unrealized losses, Available for Sale	$ 4,946	$ 4,933

Held to maturity securities continuous unrealized loss position amortized cost.

No definition available.

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

- Definition

Available for sale securities continuous unrealized loss position amortized cost.

+ References

No definition available.

+ Details

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

- Definition

Available for sale securities number.

+ References

No definition available.

+ Details

Name:	oksb_AvailableForSaleSecuritiesNumber
Namespace Prefix:	oksb_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- References

No definition available.

+ Details

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

- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27337-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.

+ References

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months.

+ References

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the fair value of securities categorized as held-to-maturity which are in an unrealized loss position.

+ References

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investments in debt securities classified as held-to-maturity that have been in a continuous loss position for less than twelve months.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Disclosure regarding factors used to determine that the impairment of securities categorized as held-to-maturity where cost exceeds fair value is not an other than temporary impairment (OTTI). This item contains disclosure of the number of investment positions in the held-to-maturity investments determined to be temporarily impaired.

+ References

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_AgencySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_AssetBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_EquitySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: