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Fair Value of Instruments - Changes in the Level 3 Fair Value Category of Unobservable Inputs (Detail) - USD ($)
$ in Thousands
 9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Beginning Balance $ 248,492  
Ending Balance 244,786  
State and Local Governments [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Beginning Balance 74,303  
Ending Balance 77,050  
Level 3 [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Beginning Balance 7,931  
Ending Balance 7,605  
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 (126) (1,619)
Payments & Maturities (200) (820)
Ending Balance 7,605 7,907
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 (256) (1,262)
Payments & Maturities (200) (820)
Ending Balance 6,182 6,720
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 130 (357)
Ending Balance $ 1,423 $ 1,187