Fair Value Measurements - Summary of Company's Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Trading securities, Amortized cost	$ 91,031	$ 93,574
Trading securities, Gross unrealized gains	1,625	233
Trading securities, Gross unrealized losses	(14)	(668)
Trading securities, Estimated fair value	92,642	93,139
Investments, Amortized cost	228,150	241,130
Investments, Gross unrealized gains	2,346	260
Investments, Gross unrealized losses	(19)	(1,285)
Investments, Estimated fair value	230,477	240,105
Corporate Debt [Member]
Schedule Of Available For Sale Securities [Line Items]
Securities available-for-sale, Amortized cost	137,119	147,556
Securities available-for-sale, Gross unrealized gains	721	27
Securities available-for-sale, Gross unrealized losses	(5)	(617)
Securities available-for-sale, Estimated fair value	137,835	146,966
Trading securities, Amortized cost	31,120	72,274
Trading securities, Gross unrealized gains	14	8
Trading securities, Gross unrealized losses	(14)	(421)
Trading securities, Estimated fair value	31,120	71,861
U.S. Treasuries [Member]
Schedule Of Available For Sale Securities [Line Items]
Trading securities, Amortized cost	54,738	16,953
Trading securities, Gross unrealized gains	567	225
Trading securities, Estimated fair value	55,305	17,178
Mutual Funds Held In Rabbi Trust [Member]
Schedule Of Available For Sale Securities [Line Items]
Trading securities, Amortized cost	5,173	4,347
Trading securities, Gross unrealized gains	1,044
Trading securities, Gross unrealized losses		(247)
Trading securities, Estimated fair value	$ 6,217	$ 4,100

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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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