Fair Value of Financial Instruments - Summary of Estimated Fair Values of Investments and Remaining Unfunded Commitments for Each Major Category of Investments (Textual) (Detail) $ in Thousands	9 Months Ended
Sep. 30, 2018 USD ($)
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value	$ 468,436
Unfunded commitments	19,084
Fair value accounting \| Venture capital and private equity fund investments \| Non-marketable securities
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value	334,419
Unfunded commitments	$ 13,255
Fair value accounting \| Venture capital and private equity fund investments \| Non-marketable securities \| Lower Limit
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Period distributions from fund investments to be received	10 years
Fair value accounting \| Venture capital and private equity fund investments \| Non-marketable securities \| Upper Limit
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Period distributions from fund investments to be received	13 years
Fair value accounting \| Venture capital and private equity fund investments \| Non-marketable securities \| Noncontrolling Interests
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value	$ 93,000
Unfunded commitments	4,300
Equity method accounting \| Venture capital and private equity fund investments \| Non-marketable securities
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value	112,537
Unfunded commitments	$ 4,943
Equity method accounting \| Venture capital and private equity fund investments \| Non-marketable securities \| Lower Limit
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Period distributions from fund investments to be received	5 years
Equity method accounting \| Venture capital and private equity fund investments \| Non-marketable securities \| Upper Limit
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Period distributions from fund investments to be received	8 years

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

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- Definition

Estimate remaining period the underlying assets are expected to be liquidated by the investees for investments for which net asset value per share is calculated (including by unit, membership interest, or other ownership interest unit measure), in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=SL6742756-110258

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- Definition

The total fair value for all investments, comprising or included in the major category, of those certain investments for which net asset value per share is calculated (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments) for which the entity has intentions to sell or liquidate, but the specific assets for sale or liquidation have not been identified (for example, the entity intends to sell $10,000,000 of its investments in private equity funds, but the individual investments to be sold have not been identified).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (h)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=SL6742756-110258

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- Definition

The amount of unfunded commitments for which the entity is obligated, to those certain investments for which net asset value per share is calculated (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=SL6742756-110258

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