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Fair Value of Instruments - Changes in the Level 3 Fair Value Category of Unobservable Inputs (Detail) - USD ($)
$ in Thousands
 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Beginning Balance $ 218,527  
Ending Balance 195,582  
State and Local Governments [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Beginning Balance 62,817  
Ending Balance 57,273  
Level 3 [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Beginning Balance 1,418  
Ending Balance 1,448  
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 30 58
Payments & Maturities   (5,904)
Ending Balance 1,448 1,506
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 30 58
Ending Balance $ 1,448 $ 1,506