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Investments - Gross Unrealized Gains and Losses and Cost of Equity (Detail) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Investment [Line Items] |  |
Equity securities, Cost or Amortized Cost | $ 19,962 | $ 8,503
Equity securities, Gross Unrealized Gains | 30 | 779
Equity securities, Gross Unrealized Losses | (1,341) |
Equity securities, Fair Value | 18,651 | $ 9,282
Domestic Common Stock [Member] |  |
Investment [Line Items] |  |
Equity securities, Cost or Amortized Cost | 19,962 |
Equity securities, Gross Unrealized Gains | 30 |
Equity securities, Gross Unrealized Losses | (1,341) |
Equity securities, Fair Value | $ 18,651 |

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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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