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Fair Value of Instruments - Changes in the Level 3 Fair Value Category of Unobservable Inputs (Detail) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Beginning Balance $ 196,398  
Ending Balance 192,859  
Level 3 [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Beginning Balance 1,428  
Ending Balance 1,423  
Level 3 [Member] | State and Local Governments [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Beginning Balance 1,428 $ 1,418
Change in Market Value (5) (4)
Payments & Maturities 0 0
Ending Balance 1,423 1,414
Level 3 [Member] | State and Local Governments [Member] | Tax - Exempt [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Payments & Maturities 0 0
Level 3 [Member] | State and Local Governments [Member] | Taxable [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Beginning Balance 1,428 1,418
Change in Market Value (5) (4)
Payments & Maturities 0 0
Ending Balance $ 1,423 $ 1,414