Stockholders' Deficit - Restricted Stock Units (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Weighted Average Grant Date Fair Value (in dollars per share) [Roll Forward]
Stock-based compensation expense recognized	$ 37.0	$ 44.0	$ 27.0
Restricted Stock Units
Number of Restricted Stock Units (in shares) [Roll Forward]
Unvested units outstanding at the beginning of the period (in shares)	2.6
Number of RSUs granted (in shares)	1.5
Number of RSUs vested (in shares)	(1.0)
Number of RSUs cancelled (in shares)	(0.2)
Unvested units outstanding at the end of the period (in shares)	2.9	2.6
Weighted Average Grant Date Fair Value (in dollars per share) [Roll Forward]
Weighted average grant date fair value of unvested units outstanding at the beginning of the period (in usd per share)	$ 25.37
Fair value of RSUs granted (in usd per share)	21.78	$ 47.17
Fair value of RSUs vested (in usd per share)	19.17
Fair value of RSUs cancelled (in usd per share)	32.26
Weighted average grant date fair value of unvested units outstanding at the end of the period (in usd per share)	$ 24.80	$ 25.37
Stock-based compensation expense recognized	$ 23.0	$ 32.0	23.0
Unrecognized stock-based compensation expense relating to unvested awards that will be amortized	$ 44.0
Weighted-average period of amortization	2 years
Fair value at vesting date	$ 22.0	$ 88.0	$ 47.0

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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Unrecognized cost of unvested share-based compensation awards.

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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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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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Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

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