Fair Value Measurements - Summary of Company's Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Schedule Of Available For Sale Securities [Line Items]
Securities available-for-sale, Amortized cost	$ 94,760	$ 84,972
Securities available-for-sale, Gross unrealized gains	59	4
Securities available-for-sale, Gross unrealized losses	(38)	(270)
Securities available-for-sale, Estimated fair value	94,781	84,706
Trading securities, Amortized cost	75,537
Trading securities, Gross unrealized gains	349
Trading securities, Gross unrealized losses	(12)
Trading securities, Estimated fair value	75,874
Investments, Amortized cost	170,297
Investments, Gross unrealized gains	408
Investments, Gross unrealized losses	(50)
Investments, Estimated fair value	170,655	84,706
Corporate Debt [Member]
Schedule Of Available For Sale Securities [Line Items]
Securities available-for-sale, Amortized cost	94,760	82,937
Securities available-for-sale, Gross unrealized gains	59	4
Securities available-for-sale, Gross unrealized losses	(38)	(270)
Securities available-for-sale, Estimated fair value	94,781	82,671
Trading securities, Amortized cost	74,370
Trading securities, Gross unrealized gains	188
Trading securities, Gross unrealized losses	(12)
Trading securities, Estimated fair value	74,546
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	1,167
Trading securities, Gross unrealized gains	161
Trading securities, Estimated fair value	$ 1,328

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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Amount of investments in debt and equity securities, including, but not limited to, held-to-maturity, trading and available-for-sale expected to be converted to cash, sold or exchanged within one year or the normal operating cycle, if longer.

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