Fair Value Measurements - Summary of Company's Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Schedule Of Available For Sale Securities [Line Items]
Trading securities, Amortized cost	$ 30,810	$ 91,031
Trading securities, Gross unrealized gains	233	1,625
Trading securities, Gross unrealized losses	231	(14)
Trading securities, Estimated fair value	31,274	92,642
Investments, Amortized cost	30,810	228,150
Investments, Gross unrealized gains	233	2,346
Investments, Gross unrealized losses	231	(19)
Investments, Estimated fair value	31,274	230,477
Corporate Debt [Member]
Schedule Of Available For Sale Securities [Line Items]
Securities available-for-sale, Amortized cost		137,119
Securities available-for-sale, Gross unrealized gains		721
Securities available-for-sale, Gross unrealized losses		(5)
Securities available-for-sale, Estimated fair value		137,835
Trading securities, Amortized cost	23,130	31,120
Trading securities, Gross unrealized gains	107	14
Trading securities, Gross unrealized losses		(14)
Trading securities, Estimated fair value	23,237	31,120
U.S. Treasuries [Member]
Schedule Of Available For Sale Securities [Line Items]
Trading securities, Amortized cost		54,738
Trading securities, Gross unrealized gains		567
Trading securities, Estimated fair value		55,305
Mutual Funds Held In Rabbi Trust [Member]
Schedule Of Available For Sale Securities [Line Items]
Trading securities, Amortized cost	7,680	5,173
Trading securities, Gross unrealized gains	126	1,044
Trading securities, Gross unrealized losses	231
Trading securities, Estimated fair value	$ 8,037	$ 6,217

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Marketable securities amortized cost.

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in marketable security.

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