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Fair Value of Instruments - Changes in the Level 3 Fair Value Category of Unobservable Inputs (Detail) - USD ($) $ in Thousands | 6 Months Ended
--- | ---
Jun. 30, 2017 | Jun. 30, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |
Beginning Balance | $ 218,527 |
Ending Balance | 195,582 |
State and Local Governments [Member] |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |
Beginning Balance | 62,817 |
Ending Balance | 57,273 |
Level 3 [Member] |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |
Beginning Balance | 1,418 |
Ending Balance | 1,448 |
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 | 30 | 58
Payments & Maturities |  | (5,904)
Ending Balance | 1,448 | 1,506
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 | 30 | 58
Ending Balance | $ 1,448 | $ 1,506

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