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Fair Value of Instruments - Changes in the Level 3 Fair Value Category of Unobservable Inputs (Detail) (USD $)
In Thousands, unless otherwise specified
 6 Months Ended 6 Months Ended
Jun. 30, 2014
Jun. 30, 2013
Jun. 30, 2014
State and local governments [Member]
Dec. 31, 2013
State and local governments [Member]
Jun. 30, 2014
Unobservable Inputs (Level 3) [Member]
State and local governments [Member]
Jun. 30, 2013
Unobservable Inputs (Level 3) [Member]
State and local governments [Member]
Jun. 30, 2014
Unobservable Inputs (Level 3) [Member]
State and local governments [Member]
Tax - Exempt [Member]
Jun. 30, 2013
Unobservable Inputs (Level 3) [Member]
State and local governments [Member]
Tax - Exempt [Member]
Jun. 30, 2014
Unobservable Inputs (Level 3) [Member]
State and local governments [Member]
Taxable [Member]
Jun. 30, 2013
Unobservable Inputs (Level 3) [Member]
State and local governments [Member]
Taxable [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]                    
Beginning Balance $ 324,509   $ 78,969 $ 81,473 $ 10,346 $ 16,408 $ 8,802 $ 14,863 $ 1,544 $ 1,545
Change in Market Value         (1,452) (115) (1,093) (116) (359) 1
Purchases 0 83,310       519   519    
Payments & Maturities         (820) (4,388) (820) (4,388)    
Ending Balance $ 284,628   $ 78,969 $ 81,473 $ 8,074 $ 12,424 $ 6,889 $ 10,878 $ 1,185 $ 1,546