Allowance for Loan Losses - Analysis of Company's TDR Agreements (Detail) $ in Thousands	12 Months Ended
Dec. 31, 2016 USD ($) Contract	Dec. 31, 2015 USD ($) Contract	Dec. 31, 2014 Contract
Financing Receivable Modifications [Line Items]
Number of Contracts \| Contract	8	8	0
Commercial and Industrial [Member]
Financing Receivable Modifications [Line Items]
Number of Contracts \| Contract	2	1
Pre-Modification Outstanding Recorded Investment	$ 2,468	$ 11
Post-Modification Outstanding Recorded Investment	$ 2,468	$ 11
CRE Owner Occupied [Member]
Financing Receivable Modifications [Line Items]
Number of Contracts \| Contract	1	2
Pre-Modification Outstanding Recorded Investment	$ 22	$ 205
Post-Modification Outstanding Recorded Investment	$ 22	$ 205
Residential Real Estate Loans [Member]
Financing Receivable Modifications [Line Items]
Number of Contracts \| Contract	5	5
Pre-Modification Outstanding Recorded Investment	$ 906	$ 739
Post-Modification Outstanding Recorded Investment	$ 913	$ 670

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.

No definition available.

+ Details

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

- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=66008989&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Post-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Pre-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_FinancialInstrumentAxis=snbc_CommercialAndIndustrialLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=snbc_CREOwnerOccupiedLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=snbc_ResidentialRealEstateLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: