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Stockholders' Equity - Equity Incentive Plans (Details) - USD ($) $ in Millions | 12 Months Ended | 40 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Dec. 31, 2018 | Dec. 31, 2018 | Mar. 22, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Number of shares reserved for future issuance (in shares) | 41,400,000 | 41,400,000 |  |  |  |
Shares issued and outstanding (in shares) | 1,300,000 | 1,300,000 |  | 5,000,000 | 5,400,000 | 13,400,000
2018 Plan |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Expiration period | 10 years |  |  |  |  |
Options granted (in shares) |  | 0 |  |  |  |
Remaining unamortized stock-based compensation | $ 429.0 | $ 429.0 |  |  |  |
Unamortized stock-based compensation, requisite service period | 2 years 9 months 18 days |  |  |  |  |
2018 Plan | Class A common stock |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Number of shares reserved for future issuance (in shares) |  |  | 41,400,000 |  |  |
2018 Plan | Employee Stock Option |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Expiration period | 10 years |  |  |  |  |
Vesting period | 4 years |  |  |  |  |
2008 Equity Incentive Plan, 2017 Equity Incentive Plan, and 2018 Plan |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Shares issued and outstanding (in shares) | 26,300,000 | 26,300,000 |  |  |  |
Shares available for grant (in shares) | 57,100,000 | 57,100,000 |  |  |  |

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- Definition 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. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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