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Fair Value Measurements (Detail) - Fair Value Measurements at Reporting Date Using - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Investment Securities	$ 672,463	$ 401,132
Short-term investments
Investment Securities	532,477	335,928
Long-term investments
Investment Securities	0	15,542
Fair Value, Inputs, Level 1
Investment Securities	5,340	42,653
Fair Value, Inputs, Level 1 | Short-term investments
Investment Securities	0	0
Fair Value, Inputs, Level 1 | Long-term investments
Investment Securities	0	0
Fair Value, Inputs, Level 2
Investment Securities	667,123	358,479
Fair Value, Inputs, Level 2 | Short-term investments
Investment Securities	532,477	335,928
Fair Value, Inputs, Level 2 | Long-term investments
Investment Securities	0	15,542
Commercial paper | Short-term investments
Investment Securities	229,821	161,286
Commercial paper | Fair Value, Inputs, Level 1 | Short-term investments
Investment Securities	0	0
Commercial paper | Fair Value, Inputs, Level 2 | Short-term investments
Investment Securities	229,821	161,286
US Government Corporations and Agencies Securities | Short-term investments
Investment Securities	48,598	13,515
US Government Corporations and Agencies Securities | Fair Value, Inputs, Level 1 | Short-term investments
Investment Securities	0	0
US Government Corporations and Agencies Securities | Fair Value, Inputs, Level 2 | Short-term investments
Investment Securities	48,598	13,515
US Treasury Securities | Short-term investments
Investment Securities	0	2,915
US Treasury Securities | Long-term investments
Investment Securities	0	146
US Treasury Securities | Fair Value, Inputs, Level 1 | Short-term investments
Investment Securities	0	0
US Treasury Securities | Fair Value, Inputs, Level 1 | Long-term investments
Investment Securities	0	0
US Treasury Securities | Fair Value, Inputs, Level 2 | Short-term investments
Investment Securities	0	2,915
US Treasury Securities | Fair Value, Inputs, Level 2 | Long-term investments
Investment Securities	0	146
Municipal debt securities | Short-term investments
Investment Securities	87,290	12,237
Municipal debt securities | Fair Value, Inputs, Level 1 | Short-term investments
Investment Securities	0	0
Municipal debt securities | Fair Value, Inputs, Level 2 | Short-term investments
Investment Securities	87,290	12,237
Corporate debt securities | Short-term investments
Investment Securities	166,768	145,975
Corporate debt securities | Long-term investments
Investment Securities	0	15,396
Corporate debt securities | Fair Value, Inputs, Level 1 | Short-term investments
Investment Securities	0	0
Corporate debt securities | Fair Value, Inputs, Level 1 | Long-term investments
Investment Securities	0	0
Corporate debt securities | Fair Value, Inputs, Level 2 | Short-term investments
Investment Securities	166,768	145,975
Corporate debt securities | Fair Value, Inputs, Level 2 | Long-term investments
Investment Securities	0	15,396
Cash Equivalents
Cash Equivalents	139,986	49,662
Cash Equivalents | Fair Value, Inputs, Level 1
Cash Equivalents	5,340	42,653
Cash Equivalents | Fair Value, Inputs, Level 2
Cash Equivalents	134,646	7,009
Cash Equivalents | Money market funds
Cash Equivalents	5,340	42,653
Cash Equivalents | Money market funds | Fair Value, Inputs, Level 1
Cash Equivalents	5,340	42,653
Cash Equivalents | Money market funds | Fair Value, Inputs, Level 2
Cash Equivalents	0	0
Cash Equivalents | Commercial paper
Cash Equivalents	48,908	5,807
Cash Equivalents | Commercial paper | Fair Value, Inputs, Level 1
Cash Equivalents	0	0
Cash Equivalents | Commercial paper | Fair Value, Inputs, Level 2
Cash Equivalents	48,908	5,807
Cash Equivalents | US Government Corporations and Agencies Securities
Cash Equivalents	51,400	0
Cash Equivalents | US Government Corporations and Agencies Securities | Fair Value, Inputs, Level 1
Cash Equivalents	0	0
Cash Equivalents | US Government Corporations and Agencies Securities | Fair Value, Inputs, Level 2
Cash Equivalents	51,400	0
Cash Equivalents | Municipal debt securities
Cash Equivalents	34,338	1,202
Cash Equivalents | Municipal debt securities | Fair Value, Inputs, Level 1
Cash Equivalents	0	0
Cash Equivalents | Municipal debt securities | Fair Value, Inputs, Level 2
Cash Equivalents	$ 34,338	$ 1,202

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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