Debt - Additional Information (Detail)				1 Months Ended		12 Months Ended
	Jul. 31, 2018 USD ($)	Jul. 03, 2018 USD ($) d $ / shares	Dec. 12, 2016 USD ($) d $ / shares	Dec. 31, 2016 USD ($)	Jan. 31, 2015	Dec. 31, 2018 USD ($) agreement d $ / shares	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2018 USD ($)	Feb. 28, 2015 USD ($)
Debt Instrument [Line Items]
Outstanding borrowings on revolving credit facility						$ 116,700,000	$ 0
Net proceeds used to purchase capped call confirmations						29,414,000	0	$ 36,616,000
Interest expense						41,255,000	27,517,000	7,408,000
Amortization of discount and issuance costs						26,672,000	18,012,000	883,000
Proceeds from issuance of Notes, net of issuance costs						364,020,000	0	447,784,000
Issuance of Convertible Senior notes, transaction costs				$ 2,494,000		2,047,000		2,494,000
Class C Capital Stock
Debt Instrument [Line Items]
Public offering price (usd per share) | $ / shares		$ 57.00
1.50% Convertible Senior Notes Due 2023
Debt Instrument [Line Items]
Equity component of convertible debt						78,600,000
Interest expense						10,100,000
Amortization of discount and issuance costs						7,300,000
Contractual coupon interest						$ 2,800,000
Effective interest rate						6.99%
Debt instrument amortization period						54 months
Debt instrument carrying value						$ 294,738,000
1.50% Convertible Senior Notes Due 2023 | Class C Capital Stock
Debt Instrument [Line Items]
Conversion price (usd per share) | $ / shares						$ 78.37
1.50% Convertible Senior Notes Due 2023 | Fair Value, Inputs, Level 3
Debt Instrument [Line Items]
Fair value of convertible notes						$ 321,900,000
1.50% Convertible Senior Notes Due 2023 | Convertible Debt
Debt Instrument [Line Items]
Debt instrument, interest rate stated percentage		1.50%
Debt instrument, principal amount issued		$ 373,800,000
Net proceeds from issuance		364,000,000.0
Net proceeds used to purchase capped call confirmations		$ 29,400,000
Debt instrument, conversion rate shares		0.0127592
Conversion price (usd per share) | $ / shares		$ 78.37
Debt instrument, convertible threshold percentage		130.00%
Debt instrument, convertible threshold trading days | d		20
Debt instrument, threshold consecutive trading days | d		30
Initial cap price (usd per share) | $ / shares		$ 105.45
Capped call confirmation premium percentage		85.00%
1.50% Convertible Senior Notes Due 2023 | Convertible Debt | Debt Instrument, Redemption, Period One
Debt Instrument [Line Items]
Repurchase price percentage of principal amount		100.00%
1.50% Convertible Senior Notes Due 2023 | Convertible Debt | Debt Instrument, Redemption, Period Two
Debt Instrument [Line Items]
Repurchase price percentage of principal amount		100.00%
1.50% Convertible Senior Notes due 2023, Over-Allotment Option | Convertible Debt
Debt Instrument [Line Items]
Debt instrument, principal amount issued		$ 48,800,000
2.00% Convertible Senior Notes Due 2021
Debt Instrument [Line Items]
Debt instrument, interest rate stated percentage			2.00%
Debt instrument, principal amount issued			$ 460,000,000.0
Net proceeds used to purchase capped call confirmations			$ 36,600,000
Repurchase price percentage of principal amount			100.00%
Interest expense						28,600,000	27,200,000	1,300,000
Amortization of discount and issuance costs						19,400,000	18,000,000.0	900,000
Contractual coupon interest						$ 9,200,000	$ 9,200,000	$ 500,000
Effective interest rate				7.44%		7.44%	7.44%	7.44%
Debt instrument amortization period						35 months
Initial cap price (usd per share) | $ / shares						$ 69.19
Capped call confirmation premium percentage						85.00%
Proceeds from issuance of Notes, net of issuance costs			$ 447,800,000
Partial repurchase of Notes				$ 370,200,000
Equity component of issuance of 2021 Notes			91,400,000
Issuance of Convertible Senior notes, transaction costs			12,200,000
Amount of fees paid			$ 11,500,000
Accrued interest						$ 800,000	$ 800,000
Debt instrument carrying value						$ 394,645,000	375,279,000
2.00% Convertible Senior Notes Due 2021 | Class C Capital Stock
Debt Instrument [Line Items]
Debt instrument, conversion rate shares						0.0190985
Conversion price (usd per share) | $ / shares			$ 52.36			$ 52.36
Debt instrument, convertible threshold percentage			130.00%
Debt instrument, convertible threshold trading days | d			20
Debt instrument, threshold consecutive trading days | d			30
2.00% Convertible Senior Notes Due 2021 | Fair Value, Inputs, Level 3
Debt Instrument [Line Items]
Fair value of convertible notes						$ 446,200,000	509,000,000.0
2.00% Convertible Senior Notes Due 2021, Over-Allotment Option
Debt Instrument [Line Items]
Debt instrument, principal amount issued			$ 60,000,000.0
2.75% Convertible Senior Notes Due 2020
Debt Instrument [Line Items]
Debt instrument, interest rate stated percentage						2.75%
Debt instrument, conversion rate shares					0.0278303
Repurchase price percentage of principal amount						100.00%
Interest expense						$ 300,000	300,000	$ 6,100,000
Debt repurchase aggregate principal amount				$ 219,900,000				$ 219,900,000
Debt instrument carrying value						$ 9,600,000	10,100,000			$ 230,000,000.0
2.75% Convertible Senior Notes Due 2020 | Class A Common Stock
Debt Instrument [Line Items]
Debt instrument, convertible threshold percentage						130.00%
Debt instrument, convertible threshold trading days | d						20
Debt instrument, threshold consecutive trading days | d						30
2.75% Convertible Senior Notes Due 2020 | Class C Common Stock Distribution | Class A Common Stock
Debt Instrument [Line Items]
Debt instrument, conversion rate shares						0.0414550
Conversion price (usd per share) | $ / shares						$ 24.12
2.75% Convertible Senior Notes Due 2020 | Fair Value, Inputs, Level 3
Debt Instrument [Line Items]
Fair value of convertible notes						$ 16,700,000	$ 17,600,000
Mortgage Lenders of America, L.L.C.
Debt Instrument [Line Items]
Number of warehouse lines of credit | agreement						2
Trulia Merger Agreement | 2.75% Convertible Senior Notes Due 2020 | Class A Common Stock
Debt Instrument [Line Items]
Debt instrument, conversion rate shares						0.0123567
Conversion price (usd per share) | $ / shares						$ 80.93
Common stock exchange ratio						0.444
Revolving Credit Facility
Debt Instrument [Line Items]
Maximum borrowing capacity						$ 500,000,000.0
Current borrowing capacity									$ 126,700,000
Revolving credit facility, initial term	1 year
Revolving credit facility, extension of initial term	2 years
Availability limit, acquisition cost of residential property	85.00%
Availability limit, market value of residential property	85.00%
Amount available for future borrowings						$ 383,300,000
Weighted average interest rate						5.86%
Revolving Credit Facility | Credit Suisse AG, Cayman Islands Branch
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 250,000,000.0					$ 500,000,000.0
Line of Credit | Warehouse Lines of Credit, due July 2019
Debt Instrument [Line Items]
Maximum borrowing capacity						50,000,000.0
Line of Credit | Mortgage Lenders of America, L.L.C. | Warehouse Lines of Credit, due March 2019
Debt Instrument [Line Items]
Maximum borrowing capacity						$ 50,000,000.0
Weighted average interest rate						4.88%
Line of Credit | Mortgage Lenders of America, L.L.C. | Warehouse Lines of Credit, due July 2019
Debt Instrument [Line Items]
Weighted average interest rate						4.73%
Line of Credit | Mortgage Lenders of America, L.L.C. | Warehouse Lines of Credit
Debt Instrument [Line Items]
Maximum borrowing capacity						$ 100,000,000.0