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
Mar. 31, 2019
Dec. 31, 2018
Mar. 31, 2018
Dec. 31, 2017
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Cash and cash equivalents $ 687,498 $ 872,237 $ 718,536 $ 736,431
Short-Term Investments 96,387 100,029    
Long-Term Investments 95,906 93,745   93,745
Adjusted Cost 879,791 1,066,011    
Fair Value 879,791 1,066,011    
Level 1 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Cash and cash equivalents 160,382 158,613    
Short-Term Investments   4,979    
Long-Term Investments 9,994 1,976   1,976
Adjusted Cost 170,376 165,568    
Fair Value 170,376 165,568    
Level 2 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Short-Term Investments 96,387 95,050    
Long-Term Investments 85,912 91,769   91,769
Adjusted Cost 182,299 186,819    
Fair Value 182,299 186,819    
Money market funds [Member] | Level 1 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Cash and cash equivalents 160,382 158,613    
Adjusted Cost 160,382 158,613    
Fair Value 160,382 158,613    
U.S. Treasury Securities | Level 1 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Short-Term Investments   4,979    
Long-Term Investments 9,994 1,976   1,976
Adjusted Cost 9,994 6,955    
Fair Value 9,994 6,955    
Mutual Funds [Member] | Level 1 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Long-Term Investments 21,905 20,957   20,957
Adjusted Cost 21,905 20,957    
Fair Value 21,905 20,957    
Corporate Notes and Bonds [Member] | Level 2 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Short-Term Investments 88,656 88,412    
Long-Term Investments 35,753 43,868   43,868
Adjusted Cost 124,409 132,280    
Fair Value 124,409 132,280    
Asset-backed Securities [Member] | Level 2 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Short-Term Investments 2,370 2,115    
Long-Term Investments 21,017 21,195   21,195
Adjusted Cost 23,387 23,310    
Fair Value 23,387 23,310    
U.S. Agency Securities | Level 2 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Short-Term Investments 5,361 4,523    
Long-Term Investments 7,237 5,749   $ 5,749
Adjusted Cost 12,598 10,272    
Fair Value 12,598 10,272    
Cash [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Cash and cash equivalents 527,116 713,624    
Fair Value $ 527,116 $ 713,624