Fair Value Measurements - Summary of Company's Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Trading securities, Amortized cost	$ 93,875	$ 94,307
Trading securities, Gross unrealized gains	17	468
Trading securities, Gross unrealized losses	(703)	(306)
Trading securities, Estimated fair value	93,189	94,469
Investments, Amortized cost	245,874	239,833
Investments, Gross unrealized gains	28	477
Investments, Gross unrealized losses	(1,631)	(789)
Investments, Estimated fair value	244,271	239,521
Corporate Debt [Member]
Schedule Of Available For Sale Securities [Line Items]
Securities available-for-sale, Amortized cost	151,999	145,526
Securities available-for-sale, Gross unrealized gains	11	9
Securities available-for-sale, Gross unrealized losses	(928)	(483)
Securities available-for-sale, Estimated fair value	151,082	145,052
Trading securities, Amortized cost	89,528	91,578
Trading securities, Gross unrealized gains	17	30
Trading securities, Gross unrealized losses	(684)	(306)
Trading securities, Estimated fair value	88,861	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	(19)
Trading securities, Estimated fair value	$ 4,328	$ 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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Total debt and equity financial instruments including: (1) securities held-to-maturity, (2) trading securities, and (3) securities available-for-sale.

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

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