Fair Value Measurements - Summary of Company's Investments (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Trading securities, Amortized cost	$ 85,315	$ 93,574
Trading securities, Gross unrealized gains	1,268	233
Trading securities, Gross unrealized losses	(49)	(668)
Trading securities, Estimated fair value	86,534	93,139
Investments, Amortized cost	230,062	241,130
Investments, Gross unrealized gains	1,949	260
Investments, Gross unrealized losses	(116)	(1,285)
Investments, Estimated fair value	231,895	240,105
Corporate Debt [Member]
Schedule Of Available For Sale Securities [Line Items]
Securities available-for-sale, Amortized cost	144,747	147,556
Securities available-for-sale, Gross unrealized gains	681	27
Securities available-for-sale, Gross unrealized losses	(67)	(617)
Securities available-for-sale, Estimated fair value	145,361	146,966
Trading securities, Amortized cost	53,188	72,274
Trading securities, Gross unrealized gains	7	8
Trading securities, Gross unrealized losses	(49)	(421)
Trading securities, Estimated fair value	53,146	71,861
U.S. Treasuries [Member]
Schedule Of Available For Sale Securities [Line Items]
Trading securities, Amortized cost	26,954	16,953
Trading securities, Gross unrealized gains	591	225
Trading securities, Estimated fair value	27,545	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	670
Trading securities, Gross unrealized losses		(247)
Trading securities, Estimated fair value	$ 5,843	$ 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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