Fair Value Measurements - Summary of Company's Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Trading securities, Amortized cost	$ 93,574	$ 94,307
Trading securities, Gross unrealized gains	233	468
Trading securities, Gross unrealized losses	(668)	(306)
Trading securities, Estimated fair value	93,139	94,469
Investments, Amortized cost	241,130	239,833
Investments, Gross unrealized gains	260	477
Investments, Gross unrealized losses	(1,285)	(789)
Investments, Estimated fair value	240,105	239,521
Corporate Debt [Member]
Schedule Of Available For Sale Securities [Line Items]
Securities available-for-sale, Amortized cost	147,556	145,526
Securities available-for-sale, Gross unrealized gains	27	9
Securities available-for-sale, Gross unrealized losses	(617)	(483)
Securities available-for-sale, Estimated fair value	146,966	145,052
Trading securities, Amortized cost	72,274	91,578
Trading securities, Gross unrealized gains	8	30
Trading securities, Gross unrealized losses	(421)	(306)
Trading securities, Estimated fair value	71,861	91,302
U.S. Treasuries [Member]
Schedule Of Available For Sale Securities [Line Items]
Trading securities, Amortized cost	16,953
Trading securities, Gross unrealized gains	225
Trading securities, Estimated fair value	17,178
Mutual Funds Held In Rabbi Trust [Member]
Schedule Of Available For Sale Securities [Line Items]
Trading securities, Amortized cost	4,347	2,729
Trading securities, Gross unrealized gains		438
Trading securities, Gross unrealized losses	(247)
Trading securities, Estimated fair value	$ 4,100	$ 3,167

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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Amortized cost of 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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