Employee Benefit Plans - Amended and Restated 2000 Employee Stock Purchase Plan - Additional Information (Detail) - ESPP Purchase Rights [Member] - USD ($) $ / shares in Units, shares in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jun. 04, 2013
Employee Stock Purchase Plan [Line Items]
Increase in the number of shares available to be sold under the ESPP				2.0
Shares issued under the ESPP	0.3	0.3	0.3
Stock issued under ESPP, average purchase price	$ 35.18	$ 32.88	$ 31.66
Total unrecognized compensation cost related to stock-based compensation arrangements granted under the ESPP	$ 1,900,000
Unrecognized compensation cost related to stock-based compensation arrangements granted under the ESPP, recognition period	1 year 9 months 18 days
Shares available for issuance under the ESPP	0.9	1.2	1.5
Minimum [Member]
Employee Stock Purchase Plan [Line Items]
Percentage of employee contribution to plan from base salary	1.00%
Maximum value of shares employees can purchase	$ 25,000
Maximum [Member]
Employee Stock Purchase Plan [Line Items]
Percentage of employee contribution to plan from base salary	10.00%
Option price as percentage of fair market value	85.00%
Maximum offering period	27 months