Stockholders' Deficit - Restricted Stock Units (Details) - Restricted Stock Units - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of Restricted Stock Units (in shares) [Roll Forward]
Unvested units outstanding at the beginning of the period (in shares)	4.9
Number of RSUs granted (in shares)	0.9
Number of RSUs vested (in shares)	(1.6)
Number of RSUs cancelled (in shares)	(0.3)
Unvested units outstanding at the end of the period (in shares)	3.9	4.9
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)	$ 11.68
Fair value of RSUs granted (in usd per share)	22.55	$ 9.35
Fair value of RSUs vested (in usd per share)	12.15
Fair value of RSUs cancelled (in usd per share)	16.25
Weighted average grant date fair value of unvested units outstanding at the end of the period (in usd per share)	$ 13.73	$ 11.68
Stock-based compensation expense recognized	$ 23.0	$ 28.8	$ 22.9
Unrecognized stock-based compensation expense relating to unvested awards that will be amortized	$ 33.2
Weighted-average period of amortization	2 years
Fair value at vesting date	$ 47.1	$ 24.2	$ 19.2

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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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

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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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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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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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