Fair Value of Financial Instruments - Schedule of Fair Value of Cash Equivalents and Marketable Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Marketable securities:
Fair Value	$ 394,548	$ 395,019
Cash Equivalents and Marketable Securities [Member]
Marketable securities:
Total cash equivalents and marketable securities	449,674	470,561
Corporate Bonds [Member]
Marketable securities:
Fair Value	250,444	282,561
Cash Equivalents [Member]
Cash equivalents:
Total cash equivalents	55,126	75,542
U.S. Government and Federal Agency Securities [Member]
Marketable securities:
Fair Value	103,213	103,466
Certificates of Deposit [Member]
Marketable securities:
Fair Value	8,815	8,992
Equity Securities [Member]
Marketable securities:
Fair Value	32,076
Money Market Mutual Funds [Member] | Cash Equivalents [Member]
Cash equivalents:
Total cash equivalents	55,126	75,542
Quoted Prices in Active Markets (Level 1) [Member]
Marketable securities:
Fair Value	32,076
Quoted Prices in Active Markets (Level 1) [Member] | Cash Equivalents and Marketable Securities [Member]
Marketable securities:
Total cash equivalents and marketable securities	32,076
Quoted Prices in Active Markets (Level 1) [Member] | Equity Securities [Member]
Marketable securities:
Fair Value	32,076
Significant Other Observable Inputs (Level 2) [Member]
Marketable securities:
Fair Value	362,472	395,019
Significant Other Observable Inputs (Level 2) [Member] | Cash Equivalents and Marketable Securities [Member]
Marketable securities:
Total cash equivalents and marketable securities	417,598	470,561
Significant Other Observable Inputs (Level 2) [Member] | Corporate Bonds [Member]
Marketable securities:
Fair Value	250,444	282,561
Significant Other Observable Inputs (Level 2) [Member] | Cash Equivalents [Member]
Cash equivalents:
Total cash equivalents	55,126	75,542
Significant Other Observable Inputs (Level 2) [Member] | U.S. Government and Federal Agency Securities [Member]
Marketable securities:
Fair Value	103,213	103,466
Significant Other Observable Inputs (Level 2) [Member] | Certificates of Deposit [Member]
Marketable securities:
Fair Value	8,815	8,992
Significant Other Observable Inputs (Level 2) [Member] | Money Market Mutual Funds [Member] | Cash Equivalents [Member]
Cash equivalents:
Total cash equivalents	$ 55,126	$ 75,542