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Fair Value of Instruments - Changes in the Level 3 Fair Value Category of Unobservable Inputs (Detail) - USD ($)
$ in Thousands
 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Beginning Balance $ 218,527  
Ending Balance 192,811  
Level 3 [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Beginning Balance 1,418  
Ending Balance 1,442  
Level 3 [Member] | State and Local Governments [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Beginning Balance 1,418 $ 7,352
Change in Market Value 24 93
Payments & Maturities   (5,904)
Ending Balance 1,442 1,541
Level 3 [Member] | State and Local Governments [Member] | Tax - Exempt [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Beginning Balance   5,904
Payments & Maturities   (5,904)
Level 3 [Member] | State and Local Governments [Member] | Taxable [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Beginning Balance 1,418 1,448
Change in Market Value 24 93
Ending Balance $ 1,442 $ 1,541