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Fair Value Measurements - Available-for-Sale Securities Measured at Fair Value (Detail) - USD ($)
$ in Thousands
Sep. 30, 2020
Dec. 31, 2019
Financial Assets:    
Restricted cash $ 2,400 $ 700
Short-term investments   491
Total available-for-sale securities 92,743 202,839
Equity securities 42,702 35,285
Fair Value, Measurements, Recurring [Member]    
Financial Assets:    
Cash and cash equivalents 410,691 229,218
Restricted cash 2,400 700
Short-term investments   491
Total available-for-sale securities 92,743 202,839
Equity securities 42,702 35,285
Fair Value, Measurements, Recurring [Member] | (Level 1) [Member]    
Financial Assets:    
Cash and cash equivalents 410,691 229,218
Restricted cash 2,400 700
Short-term investments   491
Total available-for-sale securities 85,097 193,926
Equity securities 42,702 35,285
Fair Value, Measurements, Recurring [Member] | (Level 2) [Member]    
Financial Assets:    
Total available-for-sale securities 7,646 8,913
U.S. Treasury and U.S. Government Agencies [Member] | Fair Value, Measurements, Recurring [Member]    
Financial Assets:    
Total available-for-sale securities 11,882 26,295
U.S. Treasury and U.S. Government Agencies [Member] | Fair Value, Measurements, Recurring [Member] | (Level 1) [Member]    
Financial Assets:    
Total available-for-sale securities 10,144 25,294
U.S. Treasury and U.S. Government Agencies [Member] | Fair Value, Measurements, Recurring [Member] | (Level 2) [Member]    
Financial Assets:    
Total available-for-sale securities 1,738 1,001
Corporate Bonds [Member] | Fair Value, Measurements, Recurring [Member]    
Financial Assets:    
Total available-for-sale securities 67,825 159,364
Corporate Bonds [Member] | Fair Value, Measurements, Recurring [Member] | (Level 1) [Member]    
Financial Assets:    
Total available-for-sale securities 67,825 159,364
State, Municipalities, and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member]    
Financial Assets:    
Total available-for-sale securities 5,908 7,912
State, Municipalities, and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member] | (Level 2) [Member]    
Financial Assets:    
Total available-for-sale securities 5,908 7,912
Exchange-Traded Debt [Member] | Fair Value, Measurements, Recurring [Member]    
Financial Assets:    
Total available-for-sale securities 7,095 9,145
Exchange-Traded Debt [Member] | Fair Value, Measurements, Recurring [Member] | (Level 1) [Member]    
Financial Assets:    
Total available-for-sale securities 7,095 9,145
Redeemable Preferred Stock [Member] | Fair Value, Measurements, Recurring [Member]    
Financial Assets:    
Total available-for-sale securities 33 123
Redeemable Preferred Stock [Member] | Fair Value, Measurements, Recurring [Member] | (Level 1) [Member]    
Financial Assets:    
Total available-for-sale securities $ 33 $ 123