Fair Value Measurements - Summary of Company's Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Available For Sale Securities [Line Items]
Securities available-for-sale, Amortized cost		$ 84,972
Securities available-for-sale, Gross unrealized gains		4
Securities available-for-sale, Gross unrealized losses		(270)
Securities available-for-sale, Estimated fair value		84,706
Trading securities, Amortized cost	$ 75,606
Trading securities, Gross unrealized gains	162
Trading securities, Gross unrealized losses	(234)
Trading securities, Estimated fair value	75,534
Investments, Amortized cost	194,679
Investments, Gross unrealized gains	175
Investments, Gross unrealized losses	(450)
Investments, Estimated fair value	194,404	84,706
Corporate Debt [Member]
Schedule Of Available For Sale Securities [Line Items]
Securities available-for-sale, Amortized cost	119,073	82,937
Securities available-for-sale, Gross unrealized gains	13	4
Securities available-for-sale, Gross unrealized losses	(216)	(270)
Securities available-for-sale, Estimated fair value	118,870	82,671
Trading securities, Amortized cost	74,394
Trading securities, Gross unrealized gains	47
Trading securities, Gross unrealized losses	(234)
Trading securities, Estimated fair value	74,207
Municipal Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Securities available-for-sale, Amortized cost		2,035
Securities available-for-sale, Estimated fair value		$ 2,035
Mutual Funds Held In Rabbi Trust [Member]
Schedule Of Available For Sale Securities [Line Items]
Trading securities, Amortized cost	1,212
Trading securities, Gross unrealized gains	115
Trading securities, Estimated fair value	$ 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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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 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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