Fair Value Measurements - Summary of Company's Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Trading securities, Amortized cost	$ 94,307	$ 75,606
Trading securities, Gross unrealized gains	468	162
Trading securities, Gross unrealized losses	(306)	(234)
Trading securities, Estimated fair value	94,469	75,534
Investments, Amortized cost	239,833	194,679
Investments, Gross unrealized gains	477	175
Investments, Gross unrealized losses	(789)	(450)
Investments, Estimated fair value	239,521	194,404
Corporate Debt [Member]
Schedule Of Available For Sale Securities [Line Items]
Securities available-for-sale, Amortized cost	145,526	119,073
Securities available-for-sale, Gross unrealized gains	9	13
Securities available-for-sale, Gross unrealized losses	(483)	(216)
Securities available-for-sale, Estimated fair value	145,052	118,870
Trading securities, Amortized cost	91,578	74,394
Trading securities, Gross unrealized gains	30	47
Trading securities, Gross unrealized losses	(306)	(234)
Trading securities, Estimated fair value	91,302	74,207
Mutual Funds Held In Rabbi Trust [Member]
Schedule Of Available For Sale Securities [Line Items]
Trading securities, Amortized cost	2,729	1,212
Trading securities, Gross unrealized gains	438	115
Trading securities, Estimated fair value	$ 3,167	$ 1,327

Marketable securities accumulated gross unrealized gain 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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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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