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Convertible Notes (Narrative) (Details) |  |  | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Nov. 17, 2017 USD ($) D $ / shares | Aug. 16, 2013 USD ($) D $ / shares | Sep. 30, 2018 USD ($) shares | Dec. 31, 2017 USD ($) shares | Dec. 31, 2018 USD ($) $ / shares shares | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Nov. 14, 2017 $ / shares
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Derivative, Nonmonetary Notional Amount, Shares | shares |  |  |  |  | 9,100,000 |  |  |
Class of Warrant or Right, Number of Securities Called by Warrants or Rights | shares |  |  |  |  | 9,100,000 |  |  |
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares |  |  |  |  | $ 23.30 |  |  |
Share Price | $ / shares |  |  |  |  |  |  |  | $ 14.56
Proceeds from Issuance of Warrants |  |  |  |  | $ 0 | $ 23,173,000 | $ 0 |
Potential Incremental Common Shares Attributable To Dilutive Effect Of Conversion Of Debt Securities | shares |  |  |  | 9,100,000 |  |  |  |
Additional paid in capital |  |  |  | $ 1,212,798,000 | 1,226,588,000 | 1,212,798,000 |  |
Events of default |  |  |  |  |  |  |  |
Payments for Hedge, Financing Activities |  |  |  |  | 0 | 33,523,000 | $ 0 |
Convertible notes |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Additional paid in capital |  |  |  | 111,300,000 | 111,300,000 | 111,300,000 |  |
Convertible notes | Senior, One Point Three Seven Five Percent Convertible Notes Due Two Thousand Twenty Three [Member] [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Stock Price Premium |  |  |  |  |  |  |  | 60.00%
Face Value | $ 172,500,000 |  |  | 172,500,000 | $ 172,500,000 | 172,500,000 |  |
Liability Component, Principal amount | 137,300,000 |  |  |  |  |  |  |
Equity Component, Principal amount | $ 35,200,000 |  |  |  |  |  |  |
Debt discount amortization period | 5 years |  |  |  | 4 years 1 month 2 days |  |  |
Debt Issuance Cost, Convertible, Liability Component | $ 2,600,000 |  |  |  |  |  |  |
Debt Issuance Cost, Convertible, Equity Component | 700,000 |  |  |  |  |  |  |
Debt Issuance Costs, Gross | $ 3,300,000 |  |  |  |  |  |  |
Conversion rate, number of shares to be issued per $1000 of principal (in shares) | 52.8318 |  |  |  |  |  |  |
Principal amount of notes used as the denominator to determine number of shares converted into notes | $ 1,000 |  |  |  |  |  |  |
Initial conversion price of notes (in dollars per share) | $ / shares | $ 18.93 |  |  |  |  |  |  |
Debt Instrument, Convertible, Threshold Trading Days | 20 |  |  |  |  |  |  |
Debt Instrument, Convertible, Threshold Consecutive Trading Days | D | 30 |  |  |  |  |  |  |
Debt Instrument, Convertible, Threshold Percentage of Stock Price Trigger | 130.00% |  |  |  |  |  |  |
Number of consecutive trading days before the five business days during the debt instrument measurement period | 5 days |  |  |  |  |  |  |
Denomination of the principal amount of notes used to calculate the percent of trading price during the debt instrument measurement period | $ 1,000 |  |  |  |  |  |  |
Maximum conversion price as a percentage of closing stock price | 98.00% |  |  |  |  |  |  |
Percentage of face amount of debt instrument redeemable at the company's option | 100.00% |  |  |  |  |  |  |
Events of default |  |  |  |  |  |  |  |
Period of default in payment of interest (in days) | 30 days |  |  |  |  |  |  |
Period of default to comply with other agreements (in days) | 60 days |  |  |  |  |  |  |
Minimum percentage of aggregate outstanding principal required for default event with other agreements | 25.00% |  |  |  |  |  |  |
Minimum principal amount of debt nonpayment required for debt default to occur | $ 40,000,000.0 |  |  |  |  |  |  |
Period of nonpayment of principal amount required for debt default to occur (in days) | 30 days |  |  |  |  |  |  |
Minimum percentage of aggregate outstanding principal required for nonpayment of debt default to occur | 25.00% |  |  |  |  |  |  |
Minimum percentage of aggregate outstanding principal required for immediate payment declaration to occur | 25.00% |  |  |  |  |  |  |
Debt Instrument, Default Percent Of Principal And Accrued And Unpaid Interest | 100.00% |  |  |  |  |  |  |
Convertible notes | 1.125% Convertible Senior Notes due 2018 |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Face Value |  | $ 138,000,000 |  | 81,207,000 | $ 0 | 81,207,000 |  |
Liability Component, Principal amount |  | 107,700,000 |  |  |  |  |  |
Equity Component, Principal amount |  | $ 30,300,000 |  |  |  |  |  |
Debt discount amortization period |  | 5 years |  |  |  |  |  |
Debt Issuance Cost, Convertible, Liability Component |  | $ 2,800,000 |  |  |  |  |  |
Debt Issuance Cost, Convertible, Equity Component |  | $ 800,000 |  |  |  |  |  |
Debt Instrument, Term |  | 5 years |  |  |  |  |  |
Debt Issuance Costs, Gross |  | $ 3,600,000 |  |  |  |  |  |
Conversion rate, number of shares to be issued per $1000 of principal (in shares) |  | 82.8329 |  |  |  |  |  |
Principal amount of notes used as the denominator to determine number of shares converted into notes |  | $ 1,000 |  |  |  |  |  |
Initial conversion price of notes (in dollars per share) | $ / shares |  | $ 12.07 |  |  |  |  |  |
Debt Instrument, Convertible, Threshold Trading Days |  | 20 |  |  |  |  |  |
Debt Instrument, Convertible, Threshold Consecutive Trading Days | D |  | 30 |  |  |  |  |  |
Debt Instrument, Convertible, Threshold Percentage of Stock Price Trigger |  | 130.00% |  |  |  |  |  |
Number of business days immediately after any ten consecutive trading day period during the note measurement period |  | 5 days |  |  |  |  |  |
Number of consecutive trading days before the five business days during the debt instrument measurement period |  | 5 days |  |  |  |  |  |
Denomination of the principal amount of notes used to calculate the percent of trading price during the debt instrument measurement period |  | $ 1,000 |  |  |  |  |  |
Maximum conversion price as a percentage of closing stock price |  | 98.00% |  |  |  |  |  |
Percentage of face amount of debt instrument redeemable at the company's option |  | 100.00% |  |  |  |  |  |
Debt Instrument, Repurchase Amount |  |  |  | $ 56,800,000 |  | $ 56,800,000 |  |
Events of default |  |  |  |  |  |  |  |
Period of default in payment of interest (in days) |  | 30 days |  |  |  |  |  |
Period of default to comply with other agreements (in days) |  | 60 days |  |  |  |  |  |
Minimum percentage of aggregate outstanding principal required for default event with other agreements |  | 25.00% |  |  |  |  |  |
Minimum principal amount of debt nonpayment required for debt default to occur |  | $ 40,000,000 |  |  |  |  |  |
Period of nonpayment of principal amount required for debt default to occur (in days) |  | 30 days |  |  |  |  |  |
Minimum percentage of aggregate outstanding principal required for nonpayment of debt default to occur |  | 25.00% |  |  |  |  |  |
Minimum percentage of aggregate outstanding principal required for immediate payment declaration to occur |  | 25.00% |  |  |  |  |  |
Convertible Senior Notes [Member] |  |  |  |  |  |  |  |
Events of default |  |  |  |  |  |  |  |
Repayments of Debt |  |  | $ 81,200,000 |  |  |  |  |
Debt Conversion, Converted Instrument, Shares Issued | shares |  |  | 423,873 |  |  |  |  |
Debt Instrument, Convertible, Beneficial Conversion Feature |  |  | $ 5,000,000 |  |  |  |  |
Convertible Senior Notes [Member] | Senior, One Point Three Seven Five Percent Convertible Notes Due Two Thousand Twenty Three [Member] [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Stated Interest rate (as a percent) | 1.375% |  |  |  | 1.375% |  |  |
Convertible Senior Notes [Member] | 1.125% Convertible Senior Notes due 2018 |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Stated Interest rate (as a percent) |  | 1.125% |  | 1.125% | 1.125% | 1.125% |  |

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- Definition The number of shares issued in exchange for the original debt being converted in a noncash (or part noncash) transaction. "Part noncash" refers to that portion of the transaction not resulting in cash receipts or payments in the period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4332-108586 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4304-108586 + Details
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- Definition Amount of a favorable spread to a debt holder between the amount of debt being converted and the value of the securities received upon conversion. This is an embedded conversion feature of convertible debt issued that is in-the-money at the commitment date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21538-112644 + Details
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