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Stock-Based Incentive Plans - Summary of Stock Options Granted and Outstanding (Parenthetical) (Detail) (USD $)
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Price Range $ 28.34us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice $ 36.75us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice $ 36.75us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice $ 39.70us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice
Maximum [Member] | Price Range One [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Price Range $ 87.45us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis
= snbc_PriceRangeOneMember		      
Maximum [Member] | Price Range Two [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Price Range   $ 87.45us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis
= snbc_PriceRangeTwoMember		    
Maximum [Member] | Price Range Three [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Price Range     $ 87.45us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis
= snbc_PriceRangeThreeMember		  
Minimum [Member] | Price Range One [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Price Range $ 14.25us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis
= snbc_PriceRangeOneMember		      
Minimum [Member] | Price Range Two [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Price Range   $ 14.25us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis
= snbc_PriceRangeTwoMember		    
Minimum [Member] | Price Range Three [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Price Range     $ 14.25us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis
= snbc_PriceRangeThreeMember