Fair Value Measurements - Summary of Company's Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Schedule Of Available For Sale Securities [Line Items]
Trading securities, Amortized cost	$ 36,163	$ 91,031
Trading securities, Gross unrealized gains	20	1,625
Trading securities, Gross unrealized losses	(1,354)	(14)
Trading securities, Estimated fair value	34,829	92,642
Investments, Amortized cost	136,881	228,150
Investments, Gross unrealized gains	393	2,346
Investments, Gross unrealized losses	(1,657)	(19)
Investments, Estimated fair value	135,617	230,477
Corporate Debt [Member]
Schedule Of Available For Sale Securities [Line Items]
Securities available-for-sale, Amortized cost	100,718	137,119
Securities available-for-sale, Gross unrealized gains	373	721
Securities available-for-sale, Gross unrealized losses	(303)	(5)
Securities available-for-sale, Estimated fair value	100,788	137,835
Trading securities, Amortized cost	28,483	31,120
Trading securities, Gross unrealized gains	15	14
Trading securities, Gross unrealized losses	(176)	(14)
Trading securities, Estimated fair value	28,322	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	5	1,044
Trading securities, Gross unrealized losses	(1,178)
Trading securities, Estimated fair value	$ 6,507	$ 6,217

Marketable securities accumulated gross unrealized gain before tax.

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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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