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Non-Marketable Investments - Additional Information (Detail) - USD ($) | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017 | Dec. 31, 2017
Fair Value Investments Entities That Calculate Net Asset Value Per Share [Line Items] |  |  |  |  |
Carrying value of the Company's non-marketable investments | $ 1,800,000 |  | $ 1,800,000 |  | $ 1,900,000
Losses from non-marketable investments |  |  | (100,000) | $ (200,000) |
Distributions received from funds | $ 0 | $ 0 | $ 0 | $ 400,000 |
Limited Partnerships Investments [Member] | Minimum [Member] |  |  |  |  |
Fair Value Investments Entities That Calculate Net Asset Value Per Share [Line Items] |  |  |  |  |
Ownership interest of Company |  |  | 5.00% |  |

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- Definition The net realized gains or losses on investments during the period, not including gains or losses on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.3(a)) -URI http://asc.fasb.org/extlink&oid=114873790&loc=SL114874131-224263 + Details
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- Definition Percentage of the Variable Interest Entity's (VIE) voting interest owned by (or beneficial interest in) the reporting entity (directly or indirectly). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 5A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=108788376&loc=SL6759159-111685 + Details
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