Defined contribution plan base contribution percent.

No definition available.

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Namespace Prefix:	snbc_
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- Definition

Defined contribution plan employee eligibility age.

+ References

No definition available.

+ Details

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Namespace Prefix:	snbc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Defined contribution plan employee eligibility service period.

+ References

No definition available.

+ Details

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Namespace Prefix:	snbc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The carrying amount of the liability as of the balance sheet date to an individual under a deferred compensation arrangement. This amount may be the result of periodic accruals made over the period of active employment, or reflect termination benefits resulting contractual terms or a death benefit.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 710
-SubTopic 10
-Section 30
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6409875&loc=d3e20015-108363

+ Details

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

- 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

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of discretionary contributions made by an employer to a defined contribution plan.

+ References

No definition available.

+ Details

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

- Definition

Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_PlanNameAxis=snbc_FourZeroOneKRetirementPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

Name:	us-gaap_DefinedContributionPlanTypeAxis=snbc_NextTwoPercentOfEmployeesSalaryMember
Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

Name:	us-gaap_PlanNameAxis=snbc_DirectorsPlanMember
Namespace Prefix:
Data Type:	na
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- Details

Name:	us-gaap_TitleOfIndividualAxis=us-gaap_DirectorMember
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Benefits - Additional Information (Detail) - USD ($) $ in Thousands		12 Months Ended
	Jan. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
401(k) Retirement Plan [Member]
Deferred Compensation Arrangement With Individual Share Based Payments [Line Items]
Eligible employee service completion days		90 days
Eligible employee eligibility age		21 years
Maximum employees contribution		75.00%
401(k) Retirement Plan [Member] \| Subsequent Event [Member]
Deferred Compensation Arrangement With Individual Share Based Payments [Line Items]
Company's contribution	$ 679
401(k) Retirement Plan [Member] \| First Three Percent Of Employee's Salary [Member]
Deferred Compensation Arrangement With Individual Share Based Payments [Line Items]
Percentage of employer matching contribution		100.00%
Base contribution percentage		3.00%
401(k) Retirement Plan [Member] \| Next Two Percent of Employee's Salary [Member]
Deferred Compensation Arrangement With Individual Share Based Payments [Line Items]
Percentage of employer matching contribution		50.00%
Base contribution percentage		2.00%
Directors Plan [Member] \| Director [Member]
Deferred Compensation Arrangement With Individual Share Based Payments [Line Items]
Liability under the plan		$ 1,200	$ 1,100