Fair Value Measurements - Summary of Company's Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Trading securities, Amortized cost	$ 78,256	$ 75,606
Trading securities, Gross unrealized gains	185	162
Trading securities, Gross unrealized losses	(191)	(234)
Trading securities, Estimated fair value	78,250	75,534
Investments, Amortized cost	218,731	194,679
Investments, Gross unrealized gains	198	175
Investments, Gross unrealized losses	(370)	(450)
Investments, Estimated fair value	218,559	194,404
Corporate Debt [Member]
Schedule Of Available For Sale Securities [Line Items]
Securities available-for-sale, Amortized cost	140,475	119,073
Securities available-for-sale, Gross unrealized gains	13	13
Securities available-for-sale, Gross unrealized losses	(179)	(216)
Securities available-for-sale, Estimated fair value	140,309	118,870
Trading securities, Amortized cost	75,527	74,394
Trading securities, Gross unrealized gains	68	47
Trading securities, Gross unrealized losses	(191)	(234)
Trading securities, Estimated fair value	75,404	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	117	115
Trading securities, Estimated fair value	$ 2,846	$ 1,327

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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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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