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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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