Investments - Fair Value Measurements of Cash Equivalents and Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	$ 89,068	$ 62,493
Fair Value, Measurements [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents	7,770	9,850
Available-for-sale securities	145,654	118,038
Total	153,424	127,888
Fair Value, Measurements [Member] \| Quoted Prices in Active Market for Identical Assets (Level 1) [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents	6,770	5,851
Available-for-sale securities	71,095	69,848
Total	77,865	75,699
Fair Value, Measurements [Member] \| Significant Other Observable Inputs (Level 2) [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents	1,000	3,999
Available-for-sale securities	74,559	48,190
Total	75,559	52,189
Money Market Funds [Member] \| Fair Value, Measurements [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents	6,770	5,851
Money Market Funds [Member] \| Fair Value, Measurements [Member] \| Quoted Prices in Active Market for Identical Assets (Level 1) [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents	6,770	5,851
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	48,819	32,543
Corporate Bonds [Member] \| Fair Value, Measurements [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	48,819	32,543
Corporate Bonds [Member] \| Fair Value, Measurements [Member] \| Significant Other Observable Inputs (Level 2) [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	48,819	32,543
Asset-Backed Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	13,290	6,526
Asset-Backed Bonds [Member] \| Fair Value, Measurements [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents	1,000
Available-for-sale securities	13,290	6,526
Asset-Backed Bonds [Member] \| Fair Value, Measurements [Member] \| Significant Other Observable Inputs (Level 2) [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents	1,000
Available-for-sale securities	13,290	6,526
Municipal Fixed-Rate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	2,383	2,882
Municipal Fixed-Rate Bonds [Member] \| Fair Value, Measurements [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	2,383	2,882
Municipal Fixed-Rate Bonds [Member] \| Fair Value, Measurements [Member] \| Significant Other Observable Inputs (Level 2) [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	2,383	2,882
Commercial Paper [Member] \| Fair Value, Measurements [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents		3,999
Commercial Paper [Member] \| Fair Value, Measurements [Member] \| Significant Other Observable Inputs (Level 2) [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents		3,999
Mortgage/Agency-Backed Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	8,836	5,509
Mortgage/Agency-Backed Bonds [Member] \| Fair Value, Measurements [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	8,836	5,509
Mortgage/Agency-Backed Bonds [Member] \| Fair Value, Measurements [Member] \| Significant Other Observable Inputs (Level 2) [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	8,836	5,509
U.S. Government Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	14,509	14,303
U.S. Government Bonds [Member] \| Fair Value, Measurements [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	14,509	14,303
U.S. Government Bonds [Member] \| Fair Value, Measurements [Member] \| Quoted Prices in Active Market for Identical Assets (Level 1) [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	14,509	14,303
Foreign Government Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	1,231	730
Foreign Government Bonds [Member] \| Fair Value, Measurements [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	1,231	730
Foreign Government Bonds [Member] \| Fair Value, Measurements [Member] \| Significant Other Observable Inputs (Level 2) [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	1,231	730
Marketable Equity Securities - Various Industries [Member] \| Fair Value, Measurements [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	36,519	35,662
Marketable Equity Securities - Various Industries [Member] \| Fair Value, Measurements [Member] \| Quoted Prices in Active Market for Identical Assets (Level 1) [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	36,519	35,662
Deferred Compensation Plan Assets [Member] \| Fair Value, Measurements [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	20,067	19,883
Deferred Compensation Plan Assets [Member] \| Fair Value, Measurements [Member] \| Quoted Prices in Active Market for Identical Assets (Level 1) [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	$ 20,067	$ 19,883

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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