Summary of Significant Accounting Policies - Schedule of Components of Mortgage Banking Revenue (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Accounting Policies [Abstract]
Gains on the sale of residential mortgage loans	$ 2,303us-gaap_GainLossOnSaleOfMortgageLoans	$ 12,510us-gaap_GainLossOnSaleOfMortgageLoans	$ 7,967us-gaap_GainLossOnSaleOfMortgageLoans
Gain on bulk sale of jumbo residential mortgage loans	134	1,968
Market value adjustment on loans held-for-sale	(312)us-gaap_GainLossOnSalesOfLoansNet	(1,752)us-gaap_GainLossOnSalesOfLoansNet	2,512us-gaap_GainLossOnSalesOfLoansNet
Fair value adjustment on interest rate lock commitments	(119)	(728)	847
Recourse liability provision	(460)snbc_RecourseLiabilityReserves	(400)snbc_RecourseLiabilityReserves	(775)snbc_RecourseLiabilityReserves
Mortgage banking revenue, net	$ 1,546	$ 11,598	$ 10,551

Gain on bulk sale of jumbo residential mortgage loans.

No definition available.

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- Definition

Increase decrease in fair value of interest rate lock commitments.

+ References

No definition available.

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- Definition

Mortgage banking revenue net of provision for recorded recourse liability reserve.

+ References

No definition available.

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- Definition

Recourse liability reserves.

+ References

No definition available.

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- Details

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- Definition

The gains (losses) included in earnings that represent the difference between the sale price and the carrying value of loans made to finance real estate acquisitions. This element refers to the gain (loss) and not to the cash proceeds of the sale. This element is a noncash adjustment to net income when calculating net cash generated by operating activities using the indirect method.

+ References

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- Definition

The net gain (loss) resulting from a sale of loans, including adjustments to record loans classified as held-for-sale at the lower-of-cost-or-market and fair value adjustments to loan held for investment purposes.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.13(h))

-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.3(a))

-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

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