Defined benefit plan vesting period.

No definition available.

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

- Definition

Defined contribution plan base contribution percent.

+ References

No definition available.

+ Details

Name:	snbc_DefinedContributionPlanBaseContributionPercent
Namespace Prefix:	snbc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Defined contribution plan employee eligibility age.

+ References

No definition available.

+ Details

Name:	snbc_DefinedContributionPlanEmployeeEligibilityAge
Namespace Prefix:	snbc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Defined contribution plan employee eligibility service period.

+ References

No definition available.

+ Details

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

- Definition

Retirement benefit plan eligibility period.

+ References

No definition available.

+ Details

Name:	snbc_RetirementBenefitPlanEligibilityPeriod
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

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

- Details

Name:	us-gaap_DeferredCompensationArrangementWithIndividualShareBasedPaymentsLineItems
Namespace Prefix:	us-gaap_
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

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

- Definition

Percentage of employer's matching contributions to a defined contribution plan that vests in a given year.

+ References

No definition available.

+ Details

Name:	us-gaap_DefinedContributionPlanEmployersMatchingContributionAnnualVestingPercentage
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

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

- Details

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

- Details

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

Benefits - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended			0 Months Ended
	Dec. 31, 2014 Age	Dec. 31, 2013	Dec. 31, 2012	Jan. 01, 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
Maximum employees contribution	75.00%
Percentage of employer matching contribution	50.00%
Base contribution percentage	6.00%
Company's contribution	$ 566	$ 705	$ 670
Retirement benefit plan eligibility period	1 year
Defined benefit plan vesting period	4 years
Directors Plan [Member] \| Subsequent Event [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Vesting percentage of match per year				100.00%
Directors Plan [Member] \| First Three Percent Of Employee's Salary [Member] \| Subsequent Event [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Percentage of employer matching contribution				100.00%
Base contribution percentage				3.00%
Directors Plan [Member] \| Next Two Percent of Employee's Salary [Member] \| Subsequent Event [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	$ 599	$ 522