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Investments - Fair Value Measurements of Cash Equivalents Held in Money Market Funds and Investments (Detail) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | $ 118,038 | $ 190,723
Deferred Compensation Plan Assets [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 19,883 | 14,596
Corporate Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 32,543 | 66,412
Municipal Fixed-Rate Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 2,882 | 11,774
Asset-Backed Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 6,526 | 10,206
Mortgage/Agency-Backed Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 5,509 | 13,004
U.S. Government Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 14,303 | 29,767
Foreign Government Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 730 | 3,730
Variable Rate Demand Notes [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities |  | 11,855
Fair Value, Measurements [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cash equivalents | 9,850 | 24,100
Available-for-sale securities | 118,038 | 190,723
Total | 127,888 | 214,823
Fair Value, Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cash equivalents | 5,851 | 6,878
Available-for-sale securities | 69,848 | 73,742
Total | 75,699 | 80,620
Fair Value, Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cash equivalents | 3,999 | 17,222
Available-for-sale securities | 48,190 | 116,981
Total | 52,189 | 134,203
Fair Value, Measurements [Member] | Money Market Funds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cash equivalents | 5,851 | 6,878
Fair Value, Measurements [Member] | Money Market Funds [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cash equivalents | 5,851 | 6,878
Fair Value, Measurements [Member] | Commercial Paper [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cash equivalents | 3,999 | 17,222
Fair Value, Measurements [Member] | Commercial Paper [Member] | Significant Other Observable Inputs (Level 2) [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cash equivalents | 3,999 | 17,222
Fair Value, Measurements [Member] | Deferred Compensation Plan Assets [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 19,883 | 14,596
Fair Value, Measurements [Member] | Deferred Compensation Plan Assets [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 19,883 | 14,596
Fair Value, Measurements [Member] | Corporate Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 32,543 | 66,412
Fair Value, Measurements [Member] | Corporate Bonds [Member] | Significant Other Observable Inputs (Level 2) [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 32,543 | 66,412
Fair Value, Measurements [Member] | Municipal Fixed-Rate Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 2,882 | 11,774
Fair Value, Measurements [Member] | Municipal Fixed-Rate Bonds [Member] | Significant Other Observable Inputs (Level 2) [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 2,882 | 11,774
Fair Value, Measurements [Member] | Asset-Backed Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 6,526 | 10,206
Fair Value, Measurements [Member] | Asset-Backed Bonds [Member] | Significant Other Observable Inputs (Level 2) [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 6,526 | 10,206
Fair Value, Measurements [Member] | Mortgage/Agency-Backed Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 5,509 | 13,004
Fair Value, Measurements [Member] | Mortgage/Agency-Backed Bonds [Member] | Significant Other Observable Inputs (Level 2) [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 5,509 | 13,004
Fair Value, Measurements [Member] | U.S. Government Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 14,303 | 29,767
Fair Value, Measurements [Member] | U.S. Government Bonds [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 14,303 | 29,767
Fair Value, Measurements [Member] | Foreign Government Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 730 | 3,730
Fair Value, Measurements [Member] | Foreign Government Bonds [Member] | Significant Other Observable Inputs (Level 2) [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 730 | 3,730
Fair Value, Measurements [Member] | Variable Rate Demand Notes [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities |  | 11,855
Fair Value, Measurements [Member] | Variable Rate Demand Notes [Member] | Significant Other Observable Inputs (Level 2) [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities |  | 11,855
Fair Value, Measurements [Member] | Marketable Equity Securities - Various Industries [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 35,662 | 29,379
Fair Value, Measurements [Member] | Marketable Equity Securities - Various Industries [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | $ 35,662 | $ 29,379

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