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Fair Value of Financial Instruments - Changes in the Level 3 Fair Value Category of Unobservable Inputs (Detail) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Beginning Balance $ 235,115  
Ending Balance 226,512  
State and Local Governments [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Beginning Balance 72,066  
Ending Balance 64,830  
Level 3 [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Beginning Balance 7,352  
Ending Balance 7,115  
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 33 106
Payments & Maturities (270) (200)
Ending Balance 7,115 7,837
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   (3)
Payments & Maturities (270) (200)
Ending Balance 5,634 6,435
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 33 109
Ending Balance $ 1,481 $ 1,402