Available-for-sale Securities, Gross Unrealized Gain (Loss), Number of Securities

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- Definition

Fair Value Level 1, Level 2, Level 3 Transfers, Amount

+ References

No definition available.

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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.

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- Definition

Fair value portion of borrowing which can be exchanged for a specified number of another security at the option of the issuer or the holder, for example, but not limited to, the entity's common stock.

+ References

No definition available.

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- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

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- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Amount, after accumulated amortization, of debt discount.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
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-SubTopic 30
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-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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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