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Investments - Additional Information (Detail) - USD ($) $ in Millions | 12 Months Ended |
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Dec. 31, 2018 | Jan. 01, 2018
ASU 2016-01 [Member] |  |
Schedule Of Investments [Line Items] |  |
Reclassification of net unrealized gains of marketable equity securities from AOCI to retained earnings |  | $ 3.2
Investment [Member] | Issuer Concentration [Member] | Market Value of Total Investment Portfolio [Member] |  |
Schedule Of Investments [Line Items] |  |
Investment concentration risk percentage | 5.00% |

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