Summary of Significant Accounting Policies - Additional Information (Detail)	3 Months Ended		12 Months Ended
	Mar. 31, 2017 USD ($) Segment	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($) Segment	Dec. 31, 2015 USD ($) Segment
Accounting Policies [Line Items]
Number of reportable segments | Segment	2		2	1
Software development cost capitalized	$ 2,900,000	$ 2,600,000
Goodwill impairment			$ 0	$ 0
Goodwill	1,860,291,000		1,859,909,000
Impairment of long lived assets	$ 0	1,437,000
Contract service period	36 months
Reserves for refunds and chargebacks	$ 600,000		600,000
Deferred revenue expected to be refunded	$ 2,100,000		2,100,000
Percentage of deferred revenue refunded within same fiscal month	81.00%
Percentage of deferred revenue refunded within 45 days	94.00%
Unrecognized tax benefits	$ 0	0
Interest expense and penalties	0	0
Impairment of other long-lived assets	$ 0	0
Amount of net operating loss carry-forwards available to offset future U.S. federal taxable income			142,700,000
Amount of net operating loss carry-forwards available to offset future state taxable income			$ 125,600,000
Developed Technology
Accounting Policies [Line Items]
Weighted average useful life	7 years		7 years	7 years
Webzai Limited inc | Engineering and Development
Accounting Policies [Line Items]
Impairment of long lived assets	$ 0	1,400,000
Minimum
Accounting Policies [Line Items]
Discount rate			11.00%
Maximum
Accounting Policies [Line Items]
Discount rate			12.00%
Web Presence Unit
Accounting Policies [Line Items]
Goodwill	1,255,986,000		$ 1,255,604,000
Reporting unit, percentage of fair value in excess of carrying amount			67.00%
Impairment of long lived assets	0	1,437,000
Email Marketing Unit
Accounting Policies [Line Items]
Goodwill	604,305,000		$ 604,305,000
Reporting unit, percentage of fair value in excess of carrying amount			35.00%
Impairment of long lived assets	$ 0	$ 0
Accounting Standards Update 2016-09, Excess Tax Benefit Component
Accounting Policies [Line Items]
Excess tax benefit			$ 900,000
Amount of net operating loss carry-forwards available to offset future U.S. federal taxable income				$ 1,500,000
Amount of net operating loss carry-forwards available to offset future state taxable income				$ 700,000
Cumulative effect of new accounting principle in period of adoption			$ 900,000