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Debt - Narrative (Details) - USD ($)		12 Months Ended
	Aug. 12, 2014	Dec. 31, 2017	Oct. 13, 2017	Dec. 31, 2016
Line of Credit Facility [Line Items]
Convertible Senior Notes		$ 230,000,000		$ 259,000,000
Fair value of debt		227,704,000		255,291,000
Interest rate derivative instruments, liability at fair value		200,000
2019 Notes
Line of Credit Facility [Line Items]
Convertible Senior Notes	$ 230,000,000	230,000,000		$ 230,000,000
Interest rate, as a percent	0.75%
Carrying amount of debt	$ 230,000,000	$ 227,700,000
Capitalized finance fees	$ 7,100,000
Conversion rate	0.0188072
Conversion price	$ 53.17
Repurchase price, expressed as a percentage of principal of debt repurchased	100.00%
Effective interest rate (as a percent)		1.38%
Fair value of debt		$ 218,500,000
Additional interest for non-compliance, percent		0.25%
Second additional interest for non-compliance, percent		0.50%
Percent of holders			25.00%

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Convertible Debt, Additional Interest for Non-compliance, Percent

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Convertible Debt, Second Additional Interest for Non-compliance, Percent

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Debt Instrument, Debt Default, Percent of Holders

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Represents the purchase price of debt to be repurchased expressed as a percentage of the principal amount of the repurchased debt.

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- Definition

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

+ References

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-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
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- Definition

The price per share of the conversion feature embedded in the debt instrument.

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Ratio applied to the conversion of debt instrument into equity with equity shares divided by debt principal amount.

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

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Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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

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Amount, after accumulated amortization, of debt issuance costs classified as noncurrent. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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Fair value as of the balance sheet date of all derivative liabilities not designated as hedging instruments.

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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.

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The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

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