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LOAN PORTFOLIO-Corporation's Aging of Purchased Credit Impaired Loans (Parenthetical) (Detail) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Residential Mortgage [Member]    
Accounts Notes And Loans Receivable [Line Items]    
Financing Receivable Recorded Investment 30 To 59 Days Past Due Mortgage $ 224.0 $ 142.8
Residential Mortgage [Member] | Certain Loans Acquired In Transfer Not Accounted For As Debt Securities [Member]    
Accounts Notes And Loans Receivable [Line Items]    
Financing Receivable Recorded Investment 30 To 59 Days Past Due Mortgage 28.1 22.3
Commercial Mortgage [Member]    
Accounts Notes And Loans Receivable [Line Items]    
Financing Receivable Recorded Investment 30 To 59 Days Past Due Mortgage 9.0 4.6
Commercial Mortgage [Member] | Certain Loans Acquired In Transfer Not Accounted For As Debt Securities [Member]    
Accounts Notes And Loans Receivable [Line Items]    
Financing Receivable Recorded Investment 30 To 59 Days Past Due Mortgage $ 0.2 $ 0.1