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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
 3 Months Ended 3 Months Ended
Mar. 31, 2013
Mar. 31, 2014
Dec. 31, 2013
Mar. 31, 2014
State and local governments [Member]
Dec. 31, 2013
State and local governments [Member]
Mar. 31, 2014
Unobservable Inputs (Level 3) [Member]
State and local governments [Member]
Mar. 31, 2013
Unobservable Inputs (Level 3) [Member]
State and local governments [Member]
Mar. 31, 2014
Unobservable Inputs (Level 3) [Member]
State and local governments [Member]
Tax - Exempt [Member]
Mar. 31, 2013
Unobservable Inputs (Level 3) [Member]
State and local governments [Member]
Tax - Exempt [Member]
Mar. 31, 2014
Unobservable Inputs (Level 3) [Member]
State and local governments [Member]
Taxable [Member]
Mar. 31, 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   $ 307,725 $ 324,509 $ 79,815 $ 81,473 $ 10,346 $ 16,408 $ 8,802 $ 14,863 $ 1,544 $ 1,545
Change in Market Value           (1,490) (530) (1,121) (504) (369) (26)
Purchases 54,850           519   519    
Payments & Maturities           (200) (4,388) (200) (4,388)    
Ending Balance   $ 307,725 $ 324,509 $ 79,815 $ 81,473 $ 8,656 $ 12,009 $ 7,481 $ 10,490 $ 1,175 $ 1,519