Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2018 USD ($) Segment Reporting_Unit Customer	Dec. 31, 2017 USD ($) Customer	Dec. 31, 2016 USD ($) Customer	Jan. 01, 2019 USD ($)
Significant Accounting Policies [Line Items]
Reclassifications out of accumulated other comprehensive income	$ 0	$ 0	$ 0
Number of reporting segment | Segment	1
Number of reporting unit | Reporting_Unit	1
Intangible assets with indefinite lives	$ 0	0
Depreciation	4,400,000	4,500,000	4,100,000
Write off of fully depreciated assets no longer in service	1,500,000	600,000	1,100,000
Capitalized internal-use software and website development costs	$ 3,000,000	$ 2,900,000	$ 2,800,000
Customer Concentration Risk [Member] | Accounts Receivable [Member]
Significant Accounting Policies [Line Items]
Number of customers represented 10% or more of total accounts receivable | Customer	0	1
Concentration risk, percentage	10.00%
Customer Concentration Risk [Member] | Sales Revenue, Net [Member]
Significant Accounting Policies [Line Items]
Number of customers accounted for Specific revenue | Customer	0	0	0
Minimum [Member]
Significant Accounting Policies [Line Items]
Estimated useful lives	18 months
Minimum [Member] | Accounting Standards Update 2016-02 [Member] | Subsequent Event [Member]
Significant Accounting Policies [Line Items]
Operating lease, right-of-use asset				$ 28,000,000
Operating lease, liability				28,000,000
Maximum [Member]
Significant Accounting Policies [Line Items]
Estimated useful lives	17 years
Maximum [Member] | Accounting Standards Update 2016-02 [Member] | Subsequent Event [Member]
Significant Accounting Policies [Line Items]
Operating lease, right-of-use asset				32,000,000
Operating lease, liability				$ 32,000,000