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Description of the Business and Summary of Significant Accounting Policies - Initial Public Offering and Private Placement (Details) $ / shares in Units, $ in Millions							3 Months Ended	12 Months Ended
	Apr. 03, 2018 USD ($)	Mar. 28, 2018 USD ($) $ / shares shares	Mar. 27, 2018 USD ($) $ / shares shares	Mar. 26, 2018 shares	Mar. 22, 2018 USD ($)	Mar. 07, 2018	Mar. 31, 2018 USD ($) shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Conversion of Stock [Line Items]
Reverse stock split ratio						0.6667
Stock issued in IPO (in shares)			26,822,409
Share price of stock issued in IPO (in dollars per share) | $ / shares			$ 21.00
Proceeds from initial public offering and private placement | $			$ 538.2					$ 746.6	$ 0.0	$ 0.0
Offering costs | $			$ 6.9					$ 4.5	$ 2.5	$ 0.0
Convertible preferred stock, $0.00001 par value; no shares authorized, issued and outstanding as of December 31, 2018; 151.2 shares authorized, 147.6 shares issued and outstanding with liquidation preference of $624.7 as of December 31, 2017 | Conversion of Convertible Preferred Stock to Class B Common Stock
Conversion of Stock [Line Items]
Shares converted in conversion (in shares)				147,310,563
Convertible preferred stock, $0.00001 par value; no shares authorized, issued and outstanding as of December 31, 2018; 151.2 shares authorized, 147.6 shares issued and outstanding with liquidation preference of $624.7 as of December 31, 2017 | Conversion of Convertible Preferred Stock to Class A Common Stock
Conversion of Stock [Line Items]
Shares converted in conversion (in shares)				258,620
Class B common stock
Conversion of Stock [Line Items]
Shares converted in conversion (in shares)								157,000,000
Class B common stock | Conversion of Convertible Preferred Stock to Class B Common Stock
Conversion of Stock [Line Items]
Shares issued in conversion (in shares)				147,310,563
Class B common stock | Conversion of Class B Common Stock to Class A Common Stock
Conversion of Stock [Line Items]
Shares converted in conversion (in shares)				2,609,951
Class A common stock
Conversion of Stock [Line Items]
Shares issued in conversion (in shares)								0
Class A common stock | Conversion of Convertible Preferred Stock to Class A Common Stock
Conversion of Stock [Line Items]
Shares issued in conversion (in shares)				258,620
Class A common stock | Conversion of Class B Common Stock to Class A Common Stock
Conversion of Stock [Line Items]
Shares issued in conversion (in shares)				2,609,951
Private Placement
Conversion of Stock [Line Items]
Proceeds from initial public offering and private placement | $		$ 100.0
Private Placement | Class A common stock
Conversion of Stock [Line Items]
Sale of stock (in shares)		4,761,905
Share price of stock sold in shares (in dollars per share) | $ / shares		$ 21.00
Underwriters Over-allotment Option | Class A common stock
Conversion of Stock [Line Items]
Sale of stock (in shares)		5,400,000
Share price of stock sold in shares (in dollars per share) | $ / shares		$ 21.00
Proceeds received in sale of stock | $	$ 108.4
Two-Tier RSUs
Conversion of Stock [Line Items]
Recognized cumulative unrecognized stock-based compensation | $					$ 418.7			$ 418.7
Remaining unamortized stock-based compensation | $								$ 0.1
Unamortized stock-based compensation, requisite service period								2 months 2 days
Shares released (in shares)							26,800,000
Employer related payroll expense | $							$ 13.9

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