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Fair Value of Financial Instruments - Changes in the Level 3 Fair Value Category of Unobservable Inputs (Detail) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |
Beginning Balance | $ 218,527 |  |
Purchases | 15,335 | $ 114,819 | $ 60,925
Sales | 13,562 | 85,723 | 47,029
Ending Balance | 196,398 | 218,527 |
Level 3 [Member] |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |
Beginning Balance | 1,418 |  |
Ending Balance | 1,428 | 1,418 |
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 | 10 | (294) |
Purchases | 0 | 0 |
Sales | 0 | 0 |
Payments & Maturities |  | (5,640) |
Ending Balance | 1,428 | 1,418 | 7,352
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 |
Change in Market Value |  | (264) |
Purchases | 0 | 0 |
Sales | 0 | 0 |
Payments & Maturities |  | (5,640) |
Ending Balance |  |  | 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 | 10 | (30) |
Purchases | 0 | 0 |
Sales | 0 | 0 |
Ending Balance | $ 1,428 | $ 1,418 | $ 1,448

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- Definition Amount of debt securities categorized neither as held-to-maturity nor trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -Section 50 -Paragraph 2 -Subparagraph (b)-(g) -URI http://asc.fasb.org/extlink&oid=75019621&loc=d3e62557-112803 + Details
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- Definition The cash outflow to acquire debt securities classified as available-for-sale securities, because they are not classified as either held-to-maturity securities or trading securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3213-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26853-111562 + Details
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