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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, 2017 | Sep. 30, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |
Beginning Balance | $ 218,527 |
Ending Balance | 192,811 |
Level 3 [Member] |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |
Beginning Balance | 1,418 |
Ending Balance | 1,442 |
Level 3 [Member] | State and Local Governments [Member] |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |
Beginning Balance | 1,418 | $ 7,352
Change in Market Value | 24 | 93
Payments & Maturities |  | (5,904)
Ending Balance | 1,442 | 1,541
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
Payments & Maturities |  | (5,904)
Level 3 [Member] | State and Local Governments [Member] | Taxable [Member] |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |
Beginning Balance | 1,418 | 1,448
Change in Market Value | 24 | 93
Ending Balance | $ 1,442 | $ 1,541

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