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Investments - Fair Value Measurements of Cash Equivalents and Investments (Detail) - USD ($) $ in Thousands | Mar. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | $ 35,047 | $ 40,799
Fair Value, Measurements [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cash equivalents | 7,826 | 1,554
Total debt and equity securities at fair value | 86,362 | 86,071
Total other investments held at cost | 3,375 |
Total | 97,563 | 87,625
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 | 1,279 | 1,554
Total debt and equity securities at fair value | 55,542 | 54,478
Total | 56,821 | 56,032
Fair Value, Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cash equivalents | 6,547 |
Total debt and equity securities at fair value | 30,820 | 31,593
Total | 37,367 | 31,593
Money Market Funds [Member] | Fair Value, Measurements [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cash equivalents | 1,279 | 1,554
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 | 1,279 | 1,554
Corporate Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 16,009 | 20,684
Corporate Bonds [Member] | Fair Value, Measurements [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 16,009 | 20,684
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 | 16,009 | 20,684
Commercial Paper | Fair Value, Measurements [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cash equivalents | 6,547 |
Commercial Paper | Fair Value, Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cash equivalents | 6,547 |
Municipal Fixed-Rate Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 938 | 1,313
Municipal Fixed-Rate Bonds [Member] | Fair Value, Measurements [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 938 | 1,313
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 | 938 | 1,313
Asset-Backed Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 7,175 | 5,221
Asset-Backed Bonds [Member] | Fair Value, Measurements [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 7,175 | 5,221
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 | 7,175 | 5,221
Mortgage/Agency-Backed Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 4,541 | 3,791
Mortgage/Agency-Backed Bonds [Member] | Fair Value, Measurements [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 4,541 | 3,791
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 | 4,541 | 3,791
U.S. Government Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 4,227 | 9,206
U.S. Government Bonds [Member] | Fair Value, Measurements [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 4,227 | 9,206
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 | 4,227 | 9,206
Foreign Government Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 2,157 | 584
Foreign Government Bonds [Member] | Fair Value, Measurements [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 2,157 | 584
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 | 2,157 | 584
Marketable Equity Securities - Various Industries [Member] | Fair Value, Measurements [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Marketable equity securities | 30,725 | 26,763
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] |  |
Marketable equity securities | 30,725 | 26,763
Deferred Compensation Plan Assets [Member] | Fair Value, Measurements [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Marketable equity securities | 20,416 | 18,256
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] |  |
Marketable equity securities | 20,416 | 18,256
Equity in Escrow [Member] | Fair Value, Measurements [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Marketable equity securities | 174 | 253
Equity in Escrow [Member] | Fair Value, Measurements [Member] | Quoted Prices in Active Market for Identical Assets (Level 1) [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Marketable equity securities | 174 | $ 253
Other Investments [Member] | Fair Value, Measurements [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Total other investments held at cost | $ 3,375 |

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