Allowance for Loan Losses - Additional Information (Detail) (USD $)	12 Months Ended
Dec. 31, 2014 TDRs	Dec. 31, 2013 TDRs	Dec. 31, 2012	Dec. 31, 2011
Receivables [Abstract]
Allowance for loans receivable	$ 23,246,000	$ 35,537,000	$ 45,873,000	$ 41,667,000
Ratio of allowance for loan losses to gross loans held-for-investment	1.54%	1.66%	2.02%
Loan not considered as impaired	Less than 90 days
Number of impaired loans classified as collateral TDR	3snbc_ImpairedLoansConsideredTdr
TDR with commitments to lend additional funds	1
Additional funds committed as part of TDR	$ 91,000
Number of TDR agreements that subsequently defaulted	0	0

Number of impaired loans classified as troubled debt restructures.

No definition available.

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- Definition

Loan impairment period.

+ References

No definition available.

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- Definition

Ratio of allowance for loan losses to gross loans held for investment.

+ References

No definition available.

+ Details

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- Definition

Troubled debt restructurings with commitments to lend additional funds.

+ References

No definition available.

+ Details

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- Definition

Number of financing receivables that have been modified by troubled debt restructurings within the previous 12 months and for which there was a payment default.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 34

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953682-111524

+ Details

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- Definition

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.7(d))

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 7

-Subparagraph d

-Article 9

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (g)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 30

-Section 50

-Paragraph 2

-Subparagraph (b)(1)(ii)

-URI http://asc.fasb.org/extlink&oid=6379141&loc=d3e15032-111544

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 6

-Section L

-Subsection 1

+ Details

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- Definition

Reflects the unfunded portion of contractual agreements to lend funds to borrowers for loans whose terms have been modified by troubled debt restructuring.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 40

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=25495931&loc=d3e18794-111554

+ Details

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