Fair Value Measurements - Summary of Company's Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Trading securities, Amortized cost	$ 93,510	$ 93,574
Trading securities, Gross unrealized gains	1,312	233
Trading securities, Gross unrealized losses	(9)	(668)
Trading securities, Estimated fair value	94,813	93,139
Investments, Amortized cost	233,614	241,130
Investments, Gross unrealized gains	2,044	260
Investments, Gross unrealized losses	(33)	(1,285)
Investments, Estimated fair value	235,625	240,105
Corporate Debt [Member]
Schedule Of Available For Sale Securities [Line Items]
Securities available-for-sale, Amortized cost	140,104	147,556
Securities available-for-sale, Gross unrealized gains	732	27
Securities available-for-sale, Gross unrealized losses	(24)	(617)
Securities available-for-sale, Estimated fair value	140,812	146,966
Trading securities, Amortized cost	33,676	72,274
Trading securities, Gross unrealized gains	30	8
Trading securities, Gross unrealized losses	(9)	(421)
Trading securities, Estimated fair value	33,697	71,861
U.S. Treasuries [Member]
Schedule Of Available For Sale Securities [Line Items]
Trading securities, Amortized cost	54,661	16,953
Trading securities, Gross unrealized gains	587	225
Trading securities, Estimated fair value	55,248	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	695
Trading securities, Gross unrealized losses		(247)
Trading securities, Estimated fair value	$ 5,868	$ 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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