Employee Benefit Plans and Stockholders' Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	May 28, 2015
Maximum employee contribution percentage		75.00%
Employer matching contribution percentage		50.00%
Employee contribution percentage		4.00%
Total matching contributions		$ 2.6
Number of shares authorized in ESPP		1,500,000
Maximum payroll deductions, percentage		10.00%
Employee purchase price floor, percentage		85.00%
Offering period (months)		12 months
Offering periods, frequency (months)		6 months
Number of purchase periods within an offering period		2
Purchase Period Months		6 months
Purchase date, frequency (months)		6 months
Purchases of treasury stock, shares (in shares)		800,000
Purchases of treasury stock	$ 0.0	$ 100.0	$ 0.0
Shares reserved for issuance					4,000,000
In-the-money options, maximum exercise price		$ 119.80
Options in-the-money, number		100,000
Unrecognized compensation cost		$ 126.5
Restricted Stock
Total fair value of RSUs vested	$ 144.5	$ 120.9		$ 150.0
Restricted Stock | Minimum
Vesting period, years		3 years
2015 Employee Stock Purchase Plan
Issuance of common stock for Employee Stock Purchase Plan, shares		189,904	122,857	99,192
Shares reserved for issuance		1,100,000
2005 Employee Stock Purchase Plan
Issuance of common stock for Employee Stock Purchase Plan, shares				8,539