Stockholders' Equity - Summary of RSU and RSA Award Activity (Details) - $ / shares shares in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2019	Dec. 31, 2018
Restricted Stock Units
Outstanding Restricted Stock
Outstanding Restricted Stock, Vested	(13,700)	(22,400)
Weighted-Average Grant Date Fair Value per Restricted Stock
Weighted-Average Grant Date Fair Value per Restricted Stock, Unvested Beginning Balance	$ 13.34
Weighted-Average Grant Date Fair Value per Restricted Stock, Granted	10.66
Weighted-Average Grant Date Fair Value per Restricted Stock, Vested	11.76
Weighted-Average Grant Date Fair Value per Restricted Stock, Forfeited	13.22
Weighted-Average Grant Date Fair Value per Restricted Stock, Unvested Ending Balance	$ 12.55	$ 13.34
Restricted Stock Units \| Class A Common Stock
Outstanding Restricted Stock
Outstanding Restricted Stock, Unvested Beginning Balance	149,638
Outstanding Restricted Stock, Granted	81,560
Outstanding Restricted Stock, Vested	(60,577)
Outstanding Restricted Stock, Forfeited	(19,431)
Outstanding Restricted Stock, Unvested Ending Balance	151,190	149,638
Restricted Stock Units \| Class B Common Stock
Outstanding Restricted Stock
Outstanding Restricted Stock, Unvested Beginning Balance	492
Outstanding Restricted Stock, Vested	(490)
Outstanding Restricted Stock, Forfeited	(2)
Outstanding Restricted Stock, Unvested Ending Balance		492
Restricted Stock Awards
Weighted-Average Grant Date Fair Value per Restricted Stock
Weighted-Average Grant Date Fair Value per Restricted Stock, Unvested Beginning Balance	$ 7.51
Weighted-Average Grant Date Fair Value per Restricted Stock, Granted	12.61
Weighted-Average Grant Date Fair Value per Restricted Stock, Vested	10.16
Weighted-Average Grant Date Fair Value per Restricted Stock, Forfeited	5.85
Weighted-Average Grant Date Fair Value per Restricted Stock, Unvested Ending Balance	$ 9.39	$ 7.51
Restricted Stock Awards \| Class A Common Stock
Outstanding Restricted Stock
Outstanding Restricted Stock, Unvested Beginning Balance	8,134
Outstanding Restricted Stock, Granted	3,975
Outstanding Restricted Stock, Vested	(2,160)
Outstanding Restricted Stock, Forfeited	(1,149)
Outstanding Restricted Stock, Unvested Ending Balance	8,800	8,134

Share based compensation arrangement by share based payment award equity instruments other than options rollforward.

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Share based compensation arrangement by share based payment award, equity instruments other than options, weighted average grant date fair value.

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The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iv)(3)
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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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