Summary of Changes in Fair Value of Equity Securities (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Equity Securities Fv Ni Gain Loss [Abstract]
Change in the fair value of equity securities sold during the period	$ (16,200)	$ 7,300	$ 57,700	$ 23,300
Change in the fair value of equity securities held at the end of the period	(500)	362,900	461,600	489,500
Change in the fair value of equity securities	$ (16,691)	$ 370,175	$ 519,322	$ 512,771

Amount of unrealized and realized gain (loss) on investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 321
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=116654751&loc=SL75117546-209714

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Amount of realized gain (loss) from sale of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Amount of unrealized gain (loss) on investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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