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Summary of Significant Accounting Policies - Fair Value Measurements (Details) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2018 USD ($) security | Dec. 31, 2017 USD ($) security | Jan. 31, 2018 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair value transfers in or out of Level 1, Level 2 or Level 3 | $ 0 | $ 0 |
Unamortized portion of debt discount | $ 125,002,000 |  |
Securities in an unrealized gain or loss position | security | 0 | 0 |
Convertible Notes Payable |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Unamortized portion of debt discount | $ 125,000,000.0 |  |
Face amount | $ 450,000,000.0 |  |
Convertible Notes Payable | 1.75% Convertible Senior Note Due 2025 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Proceeds from issuance of 1.75% convertible senior notes due 2025 | 1.75% |  |
Convertible notes ($450.0 face value) |  |  | $ 309,100,000
Face amount |  |  | $ 450,000,000.0
Reported Value Measurement | Fair Value, Measurements, Recurring | Level 1 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Cash and cash equivalents | $ 495,100,000 | $ 381,200,000 |
Reported Value Measurement | Fair Value, Measurements, Recurring | Level 2 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Convertible notes ($450.0 face value) | 316,600,000 |  |
Estimate of Fair Value Measurement | Fair Value, Measurements, Recurring | Level 1 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Cash and cash equivalents | 495,100,000 | 381,200,000 |
Estimate of Fair Value Measurement | Fair Value, Measurements, Recurring | Level 2 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Cash and cash equivalents | 0 | 0 |
Convertible notes ($450.0 face value) | 313,000,000.0 |  |
Estimate of Fair Value Measurement | Fair Value, Measurements, Recurring | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Cash and cash equivalents | $ 0 | $ 0 |

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- Definition Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. + References No definition available. + Details
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