Fair Value Measurements - Summary of Company's Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Trading securities, Amortized cost	$ 88,856	$ 93,574
Trading securities, Gross unrealized gains	714	233
Trading securities, Gross unrealized losses	(163)	(668)
Trading securities, Estimated fair value	89,407	93,139
Investments, Amortized cost	221,895	241,130
Investments, Gross unrealized gains	952	260
Investments, Gross unrealized losses	(389)	(1,285)
Investments, Estimated fair value	222,458	240,105
Corporate Debt [Member]
Schedule Of Available For Sale Securities [Line Items]
Securities available-for-sale, Amortized cost	133,039	147,556
Securities available-for-sale, Gross unrealized gains	238	27
Securities available-for-sale, Gross unrealized losses	(226)	(617)
Securities available-for-sale, Estimated fair value	133,051	146,966
Trading securities, Amortized cost	56,626	72,274
Trading securities, Gross unrealized gains	2	8
Trading securities, Gross unrealized losses	(163)	(421)
Trading securities, Estimated fair value	56,465	71,861
U.S. Treasuries [Member]
Schedule Of Available For Sale Securities [Line Items]
Trading securities, Amortized cost	26,949	16,953
Trading securities, Gross unrealized gains	347	225
Trading securities, Estimated fair value	27,296	17,178
Mutual Funds Held In Rabbi Trust [Member]
Schedule Of Available For Sale Securities [Line Items]
Trading securities, Amortized cost	5,281	4,347
Trading securities, Gross unrealized gains	365
Trading securities, Gross unrealized losses		(247)
Trading securities, Estimated fair value	$ 5,646	$ 4,100

Marketable securities accumulated gross unrealized gain before tax.

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Marketable securities accumulated gross unrealized loss before tax.

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Marketable securities amortized cost.

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Trading securities accumulated gross unrealized gain before tax.

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Trading securities accumulated gross unrealized loss before tax.

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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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Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading).

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