Employee Savings Plan and Employee Stock Purchase Plan - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Contribution Plan Disclosure [Line Items]
401(k) eligibility minimum service period	90 days
401(k) description of plan contributions	contribution of our employees equal to 100% of the first 1% of salary deferrals and 50% of salary deferrals between 2% and 6%, up to a maximum matching contribution of 3.5% of an employee’s salary each plan year.
Employee vested percentage in salary deferrals and roll over contributions	100.00%
Minimum period for vesting 100% contributions	2 years
Minimum period for vesting of discretionary contributions	2 years
Matching contribution	$ 6,700,000	$ 5,300,000	$ 4,100,000
Employee stock purchase plan overlapping offering period	24 months
Compensation expense related to ESPP	$ 47,268,000	$ 36,411,000	$ 36,047,000
Employee Stock Purchase Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Employees Company's common stock shares purchase limit percentage	10.00%
Employees Company's common stock shares purchase limit amount	$ 25,000
Maximum number of shares that may be purchased by a participant	2,000
Share of common stock purchase maximum	2,000,000.0
Purchase of shares of common stock	46,662	59,041	76,728
Compensation expense related to ESPP	$ 1,600,000	$ 1,300,000	$ 800,000
After Two Years Of Employment [Member]
Defined Contribution Plan Disclosure [Line Items]
Matching contributions, vesting percentage	100.00%
One Hundred Percent Match For Percent Of Participants Contribution [Member]
Defined Contribution Plan Disclosure [Line Items]
Employer contribution percentage	100.00%
Percentage of salary deferrals	1.00%
50% Matching Contribution [Member]
Defined Contribution Plan Disclosure [Line Items]
Employer contribution percentage	50.00%
Minimum [Member]
Defined Contribution Plan Disclosure [Line Items]
401(k) eligible age of employee	18 years
Minimum [Member] | 50% Matching Contribution [Member]
Defined Contribution Plan Disclosure [Line Items]
Percentage of salary deferrals	2.00%
Maximum [Member]
Defined Contribution Plan Disclosure [Line Items]
Percentage of salary deferrals	3.50%
Maximum [Member] | Employee Stock Purchase Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Purchase price of common stock expressed as a percentage of its fair market value	85.00%
Maximum [Member] | 50% Matching Contribution [Member]
Defined Contribution Plan Disclosure [Line Items]
Percentage of salary deferrals	6.00%