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Off-Balance Sheet Arrangements, Guarantees and Other Commitments - Additional Information (Detail) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2015
Dec. 31, 2016
Commitments and Contingencies Disclosure [Line Items]        
Rent expense for premises and equipment leased under operating leases $ 24.8 $ 31.3 $ 21.9  
Standby Letter of Credit        
Commitments and Contingencies Disclosure [Line Items]        
Deferred Revenue   12.0   $ 10.0
Collateral in the form of cash   $ 961.0