Stockholders' Deficit - Restricted Stock Units (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense recognized	$ 61.0	$ 37.0	$ 44.0
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense recognized	30.0	23.0	32.0
Unrecognized stock-based compensation expense relating to unvested awards that will be amortized	$ 40.0
Weighted-average period of amortization	2 years
Fair value at vesting date	$ 16.8	$ 22.0	$ 88.0
Number of Restricted Stock Units (in shares) [Roll Forward]
Unvested units outstanding at the beginning of the period (in shares)	2.9
Number of RSUs granted (in shares)	2.7
Number of RSUs vested (in shares)	(1.3)
Number of RSUs cancelled (in shares)	(1.0)
Unvested units outstanding at the end of the period (in shares)	3.3	2.9
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)	$ 24.80
Fair value of RSUs granted (in usd per share)	13.75	$ 21.78
Fair value of RSUs vested (in usd per share)	19.77
Fair value of RSUs cancelled (in usd per share)	19.25
Weighted average grant date fair value of unvested units outstanding at the end of the period (in usd per share)	$ 19.07	$ 24.80