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Investment Securities - Summary of Amortized Cost and Approximate Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands | Mar. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized Cost | $ 687,985 | $ 692,674 |
Gross Unrealized Gains | 701 | 920 |
Gross Unrealized Losses | (12,264) | (30,300) |
Fair Value | 676,422 | 663,294 |
Equity securities | 1,720 |  | $ 1,718
Investment securities, at fair value | 678,142 | 665,012 | $ 665,012
Agency-guaranteed residential mortgage-backed securities |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized Cost | 306,651 | 311,267 |
Gross Unrealized Gains | 62 | 0 |
Gross Unrealized Losses | (2,569) | (5,893) |
Fair Value | 304,144 | 305,374 |
Corporate notes |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized Cost | 381,334 | 381,407 |
Gross Unrealized Gains | 639 | 920 |
Gross Unrealized Losses | (9,695) | (24,407) |
Fair Value | $ 372,278 | $ 357,920 |

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- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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- Definition Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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- Definition Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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- Definition Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 45 -Paragraph 1A -URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740 + Details
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