Summary Of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2019 USD ($) Segment $ / shares shares	Dec. 31, 2018 USD ($) Segment $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Jul. 31, 2019 USD ($)	Jan. 01, 2019 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Gain (loss) on investments		$ 0	$ 0
Stock options, outstanding | shares	957,559	998,226
Stock options, weighted average exercise price | $ / shares	$ 30.81	$ 27.54
Common stock excluded from calculation of diluted earnings per share | shares	104,316	479,854	317,923
Convertible senior notes | shares	0	1,123,139	474,923
Number of operating segment | Segment	1
Number of reporting segment | Segment	1	2
Goodwill impairment	$ 0	$ 0	$ 0
Advertising Expense	100,000	200,000	200,000
Operating Lease, ROU assets	25,707,000
Deferred Rent Credit		4,000,000.0
Lease liabilities	30,552,000
Cash and cash equivalents	528,392,000	193,822,000	$ 173,759,000
Restricted Cash	9,000,000.0
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cash and cash equivalents	$ 415,600,000	$ 126,600,000
Accounting Standards Update 2016-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
Operating Lease, ROU assets					$ 17,000,000.0
Lease liabilities					$ 21,000,000.0
0.375% Convertible Senior Notes due 2024
Summary Of Significant Accounting Policies [Line Items]
Principal amount				$ 287,500,000
Notes, due date	Jul. 15, 2024
Notes, frequency of periodic payment	semi-annually
Total convertible senior notes				296,100,000
Fair value of convertible senior notes				$ 232,800,000
Senior convertible notes	0.375%			0.375%
Awards Granted to Executive Level Employees
Summary Of Significant Accounting Policies [Line Items]
Estimated forfeiture rates	3.00%
Awards Granted to Non-Executive Level Employees
Summary Of Significant Accounting Policies [Line Items]
Estimated forfeiture rates	8.00%
Option To Purchase Common Stock
Summary Of Significant Accounting Policies [Line Items]
Stock options, outstanding | shares	957,559	998,226	734,940
Stock options, weighted average exercise price | $ / shares		$ 27.54	$ 20.80
Restricted Stock Units (RSUs)
Summary Of Significant Accounting Policies [Line Items]
Restricted stock units, outstanding | shares	734,984	705,413	505,235
Restricted Stock Units (RSUs) | Awards Granted to Executive Level Employees
Summary Of Significant Accounting Policies [Line Items]
Estimated forfeiture rates	3.00%
Restricted Stock Units (RSUs) | Awards Granted to Non-Executive Level Employees
Summary Of Significant Accounting Policies [Line Items]
Estimated forfeiture rates	8.00%
Non-Employee Directors
Summary Of Significant Accounting Policies [Line Items]
Estimated forfeiture rates	0.00%
Employee Stock Option [Member] | Awards Granted to Executive Level Employees
Summary Of Significant Accounting Policies [Line Items]
Estimated forfeiture rates	3.00%
Employee Stock Option [Member] | Awards Granted to Non-Executive Level Employees
Summary Of Significant Accounting Policies [Line Items]
Estimated forfeiture rates	8.00%