Fair Value Measurements - Summary of Company's Investments (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Trading securities, Amortized cost	$ 99,155	$ 94,307
Trading securities, Gross unrealized gains	17	468
Trading securities, Gross unrealized losses	(600)	(306)
Trading securities, Estimated fair value	98,572	94,469
Investments, Amortized cost	251,016	239,833
Investments, Gross unrealized gains	29	477
Investments, Gross unrealized losses	(1,454)	(789)
Investments, Estimated fair value	249,591	239,521
Corporate Debt [Member]
Schedule Of Available For Sale Securities [Line Items]
Securities available-for-sale, Amortized cost	151,861	145,526
Securities available-for-sale, Gross unrealized gains	12	9
Securities available-for-sale, Gross unrealized losses	(854)	(483)
Securities available-for-sale, Estimated fair value	151,019	145,052
Trading securities, Amortized cost	94,808	91,578
Trading securities, Gross unrealized gains	17	30
Trading securities, Gross unrealized losses	(623)	(306)
Trading securities, Estimated fair value	94,202	91,302
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	23
Trading securities, Estimated fair value	$ 4,370	$ 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 of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount of investment in marketable security.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading) and investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Cost of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.

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