Loans transferred into other real estate owned.

No definition available.

Name:	snbc_LoansTransferredIntoOtherRealEstateOwned
Namespace Prefix:	snbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

number of real estate property sold.

+ References

No definition available.

+ Details

Name:	snbc_NumberOfRealEstatePropertySold
Namespace Prefix:	snbc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Real estate write down during the period.

+ References

No definition available.

+ Details

Name:	snbc_RealEstateWriteDownDuringPeriod
Namespace Prefix:	snbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The net gain (loss) resulting from sales and other disposals of real estate owned for investment purposes.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.14(d))
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

+ Details

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

- Definition

The number of real estate properties owned as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	us-gaap_NumberOfRealEstateProperties
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The number of units in a real estate property owned as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	us-gaap_NumberOfUnitsInRealEstateProperty
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The cash inflow from proceeds From Sales of Wholly Owned Real Estate and Real Estate Acquired in Settlement of Loans.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3179-108585

+ Details

Name:	us-gaap_ProceedsFromSaleOfWhollyOwnedRealEstateAndRealEstateAcquiredInSettlementOfLoans
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of real estate investments sold for entities with a substantial portion of business acquiring and holding investment real estate.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 10
-Section S99
-Paragraph 6
-Subparagraph (SX 210.5-04.(c) Schedule III)
-URI http://asc.fasb.org/extlink&oid=27047687&loc=d3e5864-122674

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 970
-SubTopic 360
-Section S99
-Paragraph 1
-Subparagraph (SX 210.12-28.4)
-URI http://asc.fasb.org/extlink&oid=6590653&loc=d3e638233-123024

+ Details

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

- Definition

Amount, before accumulated depreciation, of real estate investments for entities with a substantial portion of business acquiring and holding investment real estate.

+ References

+ Details

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

- Definition

Amount of increase (decrease) before accumulated depreciation of real estate investments for entities with a substantial portion of business acquiring and holding investment real estate.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 970
-SubTopic 360
-Section S99
-Paragraph 1
-Subparagraph (SX 210.12-28.4)
-URI http://asc.fasb.org/extlink&oid=6590653&loc=d3e638233-123024

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 10
-Section S99
-Paragraph 6
-Subparagraph (SX 210.5-04.(c) Schedule III)
-URI http://asc.fasb.org/extlink&oid=27047687&loc=d3e5864-122674

+ Details

Name:	us-gaap_RealEstatePeriodIncreaseDecrease
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_CollateralAxis=snbc_BankPropertiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementLocationAxis=us-gaap_OtherExpenseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CollateralAxis=us-gaap_CommercialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CollateralAxis=us-gaap_ResidentialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Real Estate Owned - Additional Information (Detail)	12 Months Ended
Real Estate Owned - Additional Information (Detail)	Dec. 31, 2016 USD ($) Property	Dec. 31, 2015 USD ($) Property	Dec. 31, 2014 USD ($)
Real Estate Properties [Line Items]
Real estate owned decreased	$ (281,000)
Real estate owned at carrying value	0	$ 281,000
Net loss on sales of real estate	(15,000)	43,000
Loans transferred into other real estate owned		$ 0
Real estate owned portfolio includes properties \| Property		1
Loans into real estate owned	266,000	$ 1,050,000	$ 1,554,000
Write-down of real estate owned		$ 56,000
Sale of real estate owned	$ 266,000
Number of units in real estate property maintained \| Property		1
Bank Properties [Member]
Real Estate Properties [Line Items]
Real estate owned at carrying value		$ 281,000
Number of real estate property sold \| Property	1	4
Net loss on sales of real estate	$ (15,000)
Real estate owned portfolio includes properties \| Property		4
Loans into real estate owned		$ 909,000
Sale of real estate owned	266,000	$ 950,000
Bank Properties [Member] \| Other Expense [Member]
Real Estate Properties [Line Items]
Net loss on sales of real estate	$ 15,000
Commercial Properties [Member]
Real Estate Properties [Line Items]
Number of real estate property sold \| Property		1
Sale of real estate owned		$ 90,000
Residential Properties [Member]
Real Estate Properties [Line Items]
Number of real estate property sold \| Property		1
Sale of real estate owned		$ 54,000