ACQUISITIONS (Details) £ in Millions					3 Months Ended		12 Months Ended
	Nov. 08, 2018 USD ($)	Oct. 12, 2018 GBP (£)	Oct. 12, 2018 USD ($)	Feb. 21, 2018 USD ($) shares	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jun. 01, 2015 USD ($)
Business Acquisition [Line Items]
Goodwill					$ 1,612,065,000		$ 1,611,535,000	$ 1,283,457,000
Cash paid for acquisitions, net of cash acquired					0	$ 340,074,000
Acquisitions							$ 329,938,000
ForRent, Division Of DE Holdings, Inc. [Member]
Business Acquisition [Line Items]
Aggregate purchase price				$ 376,000,000
Cash payment				$ 340,000,000
Investment owned (shares) | shares				103,280
Purchase price, shares issued				$ 36,000,000
Goodwill				266,595,000					$ 267,000,000
Goodwill, expected tax deductible amount					$ 8,000,000
Indemnification asset				6,260,000
State uncertain income tax position liability				2,047,000
Indemnification asset				5,443,000
Indemnification asset				1,000,000
Transaction costs						3,000,000
Realla Ltd [Member]
Business Acquisition [Line Items]
Cash paid for acquisitions, net of cash acquired		£ 12	$ 15,000,000
Initial payments to acquire businesses, net of cash acquired		10	13,000,000
Acquisitions		8	10,000,000
Finite-lived intangible assets acquired		£ 4	$ 5,000,000
Cozy Services Ltd [Member]
Business Acquisition [Line Items]
Cash paid for acquisitions, net of cash acquired	$ 65,000,000
Acquisitions	53,000,000
Finite-lived intangible assets acquired	$ 11,000,000
Customer Relationships [Member] | ForRent, Division Of DE Holdings, Inc. [Member]
Business Acquisition [Line Items]
Weighted-average amortization period					10 years
Database Rights [Member] | ForRent, Division Of DE Holdings, Inc. [Member]
Business Acquisition [Line Items]
Weighted-average amortization period					3 years
Acquired trade names and other intangible assets [Member]
Business Acquisition [Line Items]
Weighted-average amortization period					12 years
Acquired trade names and other intangible assets [Member] | ForRent, Division Of DE Holdings, Inc. [Member]
Business Acquisition [Line Items]
Weighted-average amortization period					10 years
Acquired building photography [Member]
Business Acquisition [Line Items]
Weighted-average amortization period					2 years
Acquired building photography [Member] | ForRent, Division Of DE Holdings, Inc. [Member]
Business Acquisition [Line Items]
Weighted-average amortization period					1 year
Maximum [Member] | ForRent, Division Of DE Holdings, Inc. [Member]
Business Acquisition [Line Items]
State uncertain income tax position liability				$ 2,000,000
Employee retention bonus						$ 11,627,000