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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

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