Branch Sale and Consolidations - Summary of Branch Assets and Liabilities Classified as Held-for-Sale (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014
Assets:
Cash	$ 1,064snbc_CashIncludedInAssetsHeldForSale
Loans held-for-sale	63,965snbc_LoansHeldForSale
Properties and equipment, net	4,035
Total branch assets held-for-sale	69,064snbc_BranchAssetsHeldForSale
Liabilities:
Deposits held-for-sale	$ 183,395snbc_BranchDepositsHeldForSale

Property plant and Equipment held-for-sale as part of the branch sale.

No definition available.

Name:	snbc_PropertyPlantAndEquipmentHeldForSale
Namespace Prefix:	snbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant