Convertible Notes, Net - Additional Information (Detail) - USD ($)	1 Months Ended	9 Months Ended
	Mar. 31, 2018	Sep. 30, 2018	Sep. 28, 2018	Dec. 31, 2017	May 31, 2017	Nov. 30, 2013
Debt Instrument [Line Items]
Accrued interest payable		$ 23,701,000		$ 14,162,000
5.25% Convertible Senior Notes [Member]
Debt Instrument [Line Items]
Description of Convertible Notes conversion		The Convertible Notes are convertible at the holders' option into shares of our class A common stock, only under specific circumstances, prior to the close of business on August 31, 2018, January 31, 2022, and December 14, 2022 for the November 2013, May 2017, and March 2018 convertible notes, respectively, at the applicable conversion rate in effect on the conversion date.
Debt instrument, conversion price			$ 33.51
Discount upon issuance of Convertible Notes						$ 9,100,000
Debt issuance costs						$ 4,100,000
Convertible Notes, assumed effective interest rate						6.50%
Convertible Senior Notes, Interest rate including amortization of discount upon issuance						7.16%
Accrued interest payable		$ 10,800,000		$ 3,700,000
4.38% Convertible Senior Notes [Member]
Debt Instrument [Line Items]
Discount upon issuance of Convertible Notes	$ 1,500,000				$ 979,000
Debt issuance costs	$ 5,200,000				$ 8,400,000
Convertible Notes, assumed effective interest rate	5.49%				4.57%
Convertible Senior Notes, Interest rate including amortization of discount upon issuance					4.91%
Convertible Notes, assumed effective interest rate	5.25%
2013 Convertible Notes [Member]
Debt Instrument [Line Items]
Convertible notes value		$ 172,000