Investments - Components of Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Cost Basis	$ 51,142	$ 79,667
Unrealized Gains	0	0
Unrealized Losses	0	(364)
Fair Value	51,142	79,303
Money Market Mutual Funds [Member]
Debt Securities, Available-for-sale [Line Items]
Cost Basis	50,290	5,601
Fair Value	50,290	5,601
US Government Agencies [Member]
Debt Securities, Available-for-sale [Line Items]
Cost Basis		7,501
Unrealized Losses		(34)
Fair Value		7,467
Corporate Bonds and Notes [Member]
Debt Securities, Available-for-sale [Line Items]
Cost Basis		63,880
Unrealized Gains		0
Unrealized Losses		(330)
Fair Value		63,550
Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Cost Basis		76,982
Unrealized Gains		0
Unrealized Losses		(364)
Fair Value		76,618
Deferred Compensation Plan Assets [Member]
Debt Securities, Available-for-sale [Line Items]
Cost Basis	852	2,685
Fair Value	$ 852	$ 2,685

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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-Section 50
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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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+ References

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-Publisher FASB
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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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-Publisher FASB
-Name Accounting Standards Codification
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