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Certain Regulatory Requirements and Capital Ratios - Additional Information (Detail) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]    
Average deposited in a special reserve account for the exclusive benefit of customers pursuant to SEC Regulations $ 1,700,000,000us-gaap_CashReserveDepositRequiredAndMade $ 1,600,000,000us-gaap_CashReserveDepositRequiredAndMade
Trust Preferred Securities    
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]    
Tier I capital (to risk-weighted assets) $ 60,000,000us-gaap_TierOneRiskBasedCapital
/ us-gaap_StatementClassOfStockAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember		  
TruPS as a basis point percentage of risk-weighted assets 5