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Fair Value of Financial Instruments - Changes in the Level 3 Fair Value Category of Unobservable Inputs (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]      
Beginning Balance $ 235,115    
Purchases 114,819 $ 60,925  
Sales 85,723 47,029 $ 57,928
Ending Balance 218,527 235,115  
State and Local Governments [Member]      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]      
Beginning Balance 72,066    
Ending Balance 62,817 72,066  
Level 3 [Member]      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]      
Beginning Balance 7,352    
Ending Balance 1,418 7,352  
Level 3 [Member] | State and Local Governments [Member]      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]      
Beginning Balance 7,352 7,931  
Change in Market Value (294) (104)  
Purchases 0 0  
Sales 0 0  
Payments & Maturities (5,640) (475)  
Ending Balance 1,418 7,352 7,931
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 6,638  
Change in Market Value (264) (259)  
Purchases 0 0  
Sales 0 0  
Payments & Maturities (5,640) (475)  
Ending Balance   5,904 6,638
Level 3 [Member] | State and Local Governments [Member] | Taxable [Member]      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]      
Beginning Balance 1,448 1,293  
Change in Market Value (30) 155  
Purchases 0 0  
Sales 0 0  
Ending Balance $ 1,418 $ 1,448 $ 1,293