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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, 2017
Dec. 31, 2016
Dec. 31, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]      
Beginning Balance $ 218,527    
Purchases 15,335 $ 114,819 $ 60,925
Sales 13,562 85,723 47,029
Ending Balance 196,398 218,527  
Level 3 [Member]      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]      
Beginning Balance 1,418    
Ending Balance 1,428 1,418  
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 10 (294)  
Purchases 0 0  
Sales 0 0  
Payments & Maturities   (5,640)  
Ending Balance 1,428 1,418 7,352
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  
Change in Market Value   (264)  
Purchases 0 0  
Sales 0 0  
Payments & Maturities   (5,640)  
Ending Balance     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 10 (30)  
Purchases 0 0  
Sales 0 0  
Ending Balance $ 1,428 $ 1,418 $ 1,448