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Investments - Fair Value Measurements of Cash Equivalents Held in Money Market Funds and Investments (Detail) - USD ($) $ in Thousands | Sep. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | $ 140,025 | $ 190,723
Fair Value, Measurements [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cash equivalents | 43,551 | 24,100
Available-for-sale securities | 140,025 | 190,723
Total | 183,576 | 214,823
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 | 8,115 | 6,878
Available-for-sale securities | 74,089 | 73,742
Total | 82,204 | 80,620
Fair Value, Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cash equivalents | 35,436 | 17,222
Available-for-sale securities | 65,936 | 116,981
Total | 101,372 | 134,203
Money Market Funds [Member] | Fair Value, Measurements [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cash equivalents | 8,115 | 6,878
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 | 8,115 | 6,878
Commercial Paper [Member] | Fair Value, Measurements [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cash equivalents | 35,436 | 17,222
Commercial Paper [Member] | Fair Value, Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cash equivalents | 35,436 | 17,222
Deferred Compensation Plan Assets [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 19,113 | 14,596
Deferred Compensation Plan Assets [Member] | Fair Value, Measurements [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 19,113 | 14,596
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 | 19,113 | 14,596
Corporate Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 45,219 | 66,412
Corporate Bonds [Member] | Fair Value, Measurements [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 45,219 | 66,412
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 | 45,219 | 66,412
Municipal Fixed-Rate Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 4,874 | 11,774
Municipal Fixed-Rate Bonds [Member] | Fair Value, Measurements [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 4,874 | 11,774
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 | 4,874 | 11,774
Asset-Backed Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 7,785 | 10,206
Asset-Backed Bonds [Member] | Fair Value, Measurements [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 7,785 | 10,206
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 | 7,785 | 10,206
Mortgage/Agency-Backed Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 7,330 | 13,004
Mortgage/Agency-Backed Bonds [Member] | Fair Value, Measurements [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 7,330 | 13,004
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 | 7,330 | 13,004
U.S. Government Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 20,894 | 29,767
U.S. Government Bonds [Member] | Fair Value, Measurements [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 20,894 | 29,767
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 | 20,894 | 29,767
Foreign Government Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 728 | 3,730
Foreign Government Bonds [Member] | Fair Value, Measurements [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 728 | 3,730
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 | 728 | 3,730
Variable Rate Demand Notes [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities |  | 11,855
Variable Rate Demand Notes [Member] | Fair Value, Measurements [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities |  | 11,855
Variable Rate Demand Notes [Member] | Fair Value, Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities |  | 11,855
Marketable Equity Securities - Various Industries [Member] | Fair Value, Measurements [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | 34,082 | 29,379
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 | $ 34,082 | $ 29,379

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