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Investments - Fair Value Measurements of Cash Equivalents and Investments (Detail) - USD ($) $ in Thousands | Jun. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | $ 65,307 | $ 62,493
Fair Value, Measurements [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cash equivalents | 2,358 | 9,850
Available-for-sale securities | 123,286 | 118,038
Total | 125,644 | 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 | 2,008 | 5,851
Available-for-sale securities | 66,828 | 69,848
Total | 68,836 | 75,699
Fair Value, Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cash equivalents | 350 | 3,999
Available-for-sale securities | 56,458 | 48,190
Total | 56,808 | 52,189
Money Market Funds [Member] | Fair Value, Measurements [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cash equivalents | 2,008 | 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 | 2,008 | 5,851
Corporate Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 35,673 | 32,543
Corporate Bonds [Member] | Fair Value, Measurements [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 35,673 | 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 | 35,673 | 32,543
Foreign Government Securities [Member] | Fair Value, Measurements [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cash equivalents | 350 |
Foreign Government Securities [Member] | Fair Value, Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cash equivalents | 350 |
Municipal Fixed-Rate Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 2,221 | 2,882
Municipal Fixed-Rate Bonds [Member] | Fair Value, Measurements [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 2,221 | 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 | 2,221 | 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
Asset-Backed Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 10,319 | 6,526
Asset-Backed Bonds [Member] | Fair Value, Measurements [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 10,319 | 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] |  |
Available-for-sale securities | 10,319 | 6,526
Mortgage/Agency-Backed Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 7,207 | 5,509
Mortgage/Agency-Backed Bonds [Member] | Fair Value, Measurements [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 7,207 | 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 | 7,207 | 5,509
U.S. Government Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 8,849 | 14,303
U.S. Government Bonds [Member] | Fair Value, Measurements [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 8,849 | 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 | 8,849 | 14,303
Foreign Government Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 1,038 | 730
Foreign Government Bonds [Member] | Fair Value, Measurements [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 1,038 | 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 | 1,038 | 730
Marketable Equity Securities - Various Industries [Member] | Fair Value, Measurements [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 37,600 | 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 | 37,600 | 35,662
Deferred Compensation Plan Assets [Member] | Fair Value, Measurements [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 20,379 | 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 | $ 20,379 | $ 19,883

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