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Off-Balance Sheet Arrangements, Guarantees and Other Commitments - Summary of Commercial and Standby Letters of Credit (Additional Information) (Detail) - USD ($)
$ in Millions
6 Months Ended
Jun. 30, 2015
Dec. 31, 2014
Venture capital and private equity fund investments    
Commitments and Contingencies Disclosure [Line Items]    
Commitments to invest, period from the inception of the fund 10 years  
Call unrestricted, percentage of committed capital 100.00%  
Venture capital and private equity fund investments | Lower Limit    
Commitments and Contingencies Disclosure [Line Items]    
Expected commitment period to invest in venture capital and private equity funds (in years) 5 years  
Venture capital and private equity fund investments | Upper Limit    
Commitments and Contingencies Disclosure [Line Items]    
Expected commitment period to invest in venture capital and private equity funds (in years) 7 years  
Standby Letter of Credit    
Commitments and Contingencies Disclosure [Line Items]    
Deferred fees $ 8.6 $ 8.4
Collateral in the form of cash 537.3  
Collateral in the form of investment securities $ 1.2