Employee Benefit Plan - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Contribution Plan Disclosure [Line Items]
Company's expense related to its defined contribution 401(k) retirement plans	$ 16.0	$ 12.3	$ 10.1
Maximum \| Zillow Merger
Defined Contribution Plan Disclosure [Line Items]
Company's contribution based on employee contribution	4.00%

Amount of cost for defined contribution plan.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 70
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration