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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, 2015
Dec. 31, 2014
Dec. 31, 2013
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]      
Beginning Balance $ 248,492    
Purchases 60,925   $ 103,320
Sales 47,029 $ 57,928 91,039
Ending Balance 235,115 248,492  
State and Local Governments [Member]      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]      
Beginning Balance 74,303    
Ending Balance 72,066 74,303  
Level 3 [Member]      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]      
Beginning Balance 7,931    
Ending Balance 7,352 7,931  
Level 3 [Member] | State and Local Governments [Member]      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]      
Beginning Balance 7,931 10,346  
Change in Market Value (104) (1,355)  
Purchases 0 0  
Sales 0 0  
Payments & Maturities (475) (1,060)  
Ending Balance 7,352 7,931 10,346
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 6,638 8,802  
Change in Market Value (259) (1,104)  
Purchases 0 0  
Sales 0 0  
Payments & Maturities (475) (1,060)  
Ending Balance 5,904 6,638 8,802
Level 3 [Member] | State and Local Governments [Member] | Taxable [Member]      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]      
Beginning Balance 1,293 1,544  
Change in Market Value 155 (251)  
Purchases 0 0  
Sales 0 0  
Ending Balance $ 1,448 $ 1,293 $ 1,544