Quarterly report pursuant to Section 13 or 15(d)

Cash, Cash Equivalents, Short-Term and Long-Term Investments - Summary of Cash, Cash Equivalents, Short-Term and Long-Term Investments by Significant Investment Category (Detail)

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Cash, Cash Equivalents, Short-Term and Long-Term Investments - Summary of Cash, Cash Equivalents, Short-Term and Long-Term Investments by Significant Investment Category (Detail) - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Jun. 30, 2018
Dec. 31, 2017
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Cash and cash equivalents $ 779,281 $ 872,237 $ 844,847 $ 736,431
Short-Term Investments 104,271 100,029    
Long-Term Investments 89,486 93,745   93,745
Adjusted Cost 973,038 1,066,011    
Fair Value 973,038 1,066,011    
Level 1 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Cash and cash equivalents 163,408 158,613    
Short-Term Investments 2,382 4,979    
Long-Term Investments 5,032 1,976   1,976
Adjusted Cost 170,822 165,568    
Fair Value 170,822 165,568    
Level 2 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Short-Term Investments 101,889 95,050    
Long-Term Investments 84,454 91,769   91,769
Adjusted Cost 186,343 186,819    
Fair Value 186,343 186,819    
Money market funds [Member] | Level 1 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Cash and cash equivalents 163,408 158,613    
Adjusted Cost 163,408 158,613    
Fair Value 163,408 158,613    
U.S. Treasury Securities | Level 1 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Short-Term Investments 2,382 4,979    
Long-Term Investments 5,032 1,976   1,976
Adjusted Cost 7,414 6,955    
Fair Value 7,414 6,955    
Mutual Funds [Member] | Level 1 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Long-Term Investments 22,334 20,957   20,957
Adjusted Cost 22,334 20,957    
Fair Value 22,334 20,957    
Corporate Notes and Bonds [Member] | Level 2 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Short-Term Investments 93,620 88,412    
Long-Term Investments 37,652 43,868   43,868
Adjusted Cost 131,272 132,280    
Fair Value 131,272 132,280    
Asset-backed Securities [Member] | Level 2 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Short-Term Investments 4,281 2,115    
Long-Term Investments 14,615 21,195   21,195
Adjusted Cost 18,896 23,310    
Fair Value 18,896 23,310    
U.S. Agency Securities | Level 2 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Short-Term Investments 3,988 4,523    
Long-Term Investments 9,853 5,749   $ 5,749
Adjusted Cost 13,841 10,272    
Fair Value 13,841 10,272    
Cash [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Cash and cash equivalents 615,873 713,624    
Fair Value $ 615,873 $ 713,624