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Convertible Senior Notes - Narrative (Details) |  |  | 1 Months Ended | 2 Months Ended | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Jun. 30, 2018 USD ($) | May 22, 2018 USD ($) $ / shares shares | Jun. 19, 2018 USD ($) $ / shares | May 31, 2018 USD ($) | Aug. 31, 2014 USD ($) d $ / shares shares | Apr. 30, 2018 USD ($) | Sep. 30, 2018 USD ($) $ / shares | Sep. 30, 2018 USD ($) $ / shares | Dec. 31, 2017 USD ($) | May 22, 2015 USD ($)
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Carrying value of equity component of convertible notes |  |  |  |  |  |  | $ 0 | $ 0 | $ 18,859,000 |
Derivative asset |  |  |  |  |  |  | 509,257,000 | 509,257,000 | 0 |
Convertible Notes | 2019 Convertible Senior Notes |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Aggregate principal amount outstanding |  |  |  |  | $ 245,000,000.0 |  | 223,215,000 | 223,215,000 | 245,000,000 |
Initial conversion rate (shares per $1,000) |  |  |  |  | 0.0133251 |  |  |  |  |
Initial conversion price (USD per share) | $ / shares |  |  |  |  | $ 75.05 |  |  |  |  |
Interest rate (as a percent) |  |  |  |  | 0.75% |  |  |  |  |
If-converted value in excess of principal | $ 563,200,000 | $ 341,600,000 |  |  |  |  |  |  |  |
Increase (decrease) in if-converted value in excess of principal |  |  |  |  |  |  | 161,900,000 | $ 221,600,000 |  |
Notices for conversion |  |  |  |  |  | $ 21,800,000 | $ 195,900,000 |  |  |
Conversion value over the principal amount |  |  |  |  |  | 31,600,000 |  |  |  |
Gain (loss) on extinguishment of debt |  |  |  |  |  | $ (600,000) |  |  |  |
Warrants issued in public offering (shares) | shares |  |  |  |  | 3,264,643 |  |  |  |  |
Exercise price of convertible bond hedge (USD per share) | $ / shares |  | $ 75.05 |  |  | $ 75.05 |  | $ 75.05 | $ 75.05 |  |
Payments for convertible bond hedges |  |  |  |  | $ 48,100,000 |  |  |  |  |
Bond hedge settlement proceeds |  |  |  |  |  |  | $ 52,100,000 |  |  |
Derivative asset |  | $ 340,000,000.0 |  |  |  |  | 561,400,000 | $ 561,400,000 |  |
Warrant exercise price (USD per share) | $ / shares |  |  |  |  | $ 125.08 |  |  |  |  |
Value of warrants issued |  |  |  |  | $ 11,600,000 |  |  |  |  |
Convertible Notes | 2019 Convertible Senior Notes | Debt Instrument, Redemption, Period One |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Threshold trading days | d |  |  |  |  | 20 |  |  |  |  |
Consecutive trading days | d |  |  |  |  | 30 |  |  |  |  |
Stock price trigger to classify convertible debt as current (as a percent) |  |  |  |  | 130.00% |  |  | 130.00% |  |
Convertible Notes | 2019 Convertible Senior Notes | Debt Instrument, Redemption, Period Two |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Threshold trading days | d |  |  |  |  | 5 |  |  |  |  |
Consecutive trading days | d |  |  |  |  | 10 |  |  |  |  |
Maximum threshold of debt trading price trigger (as a percent) |  |  |  |  | 98.00% |  |  |  |  |
Convertible Notes | 2023 Convertible Senior Notes |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Aggregate principal amount outstanding |  | $ 750,000,000 |  |  |  |  | 750,000,000 | $ 750,000,000 | 0 |
Initial conversion rate (shares per $1,000) |  |  |  | 0.0040244 |  |  |  |  |  |
Initial conversion price (USD per share) | $ / shares |  | $ 248.48 |  |  |  |  |  |  |  |
Interest rate (as a percent) |  | 0.75% |  |  |  |  |  |  |  |
Proceeds from debt, net of issuance costs |  | $ 733,100,000 |  |  |  |  |  |  |  |
Carrying value of equity component of convertible notes |  |  |  |  |  |  | 143,986,000 | 143,986,000 | $ 0 |
Warrants issued in public offering (shares) | shares |  | 3,018,327 |  |  |  |  |  |  |  |
Exercise price of convertible bond hedge (USD per share) | $ / shares |  | $ 248.48 | $ 248.48 |  |  |  |  |  |  |
Payments for convertible bond hedges |  |  |  | $ 140,300,000 |  |  |  |  |  |
Warrant exercise price (USD per share) | $ / shares |  | $ 315.38 |  |  |  |  |  |  |  |
Value of warrants issued |  |  |  | $ 90,000,000.0 |  |  |  |  |  |
Fair value of of the conversion option derivative liability |  |  |  |  |  |  |  |  |  | $ 144,000,000.0
Interest expense resulting from change in fair value |  |  | $ 13,500,000 |  |  |  | 19,200,000 | 7,500,000 |  |
Debt issuance costs |  | $ 16,900,000 |  |  |  |  |  |  |  |
Debt issuance costs related to the equity component of convertible debt |  | 3,200,000 |  |  |  |  |  |  |  |
Debt issuance costs related to the liability component of convertible debt |  | $ 13,700,000 |  |  |  |  |  |  |  |
Convertible Notes | 2023 Convertible Senior Notes | Debt Instrument, Redemption, Period One |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Threshold trading days | d |  |  |  |  | 20 |  |  |  |  |
Consecutive trading days | d |  |  |  |  | 30 |  |  |  |  |
Stock price trigger to classify convertible debt as current (as a percent) |  |  |  |  | 130.00% |  |  |  |  |
Convertible Notes | 2023 Convertible Senior Notes | Debt Instrument, Redemption, Period Two |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Threshold trading days | d |  |  |  |  | 5 |  |  |  |  |
Consecutive trading days | d |  |  |  |  | 10 |  |  |  |  |
Maximum threshold of debt trading price trigger (as a percent) |  |  |  |  | 98.00% |  |  |  |  |
Measurement Input, Discount Rate | Convertible Notes | 2019 Convertible Senior Notes |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Discount rate (as a percent) |  |  |  |  | 0.0583 |  |  |  |  |
Other Expense, Net | Convertible Notes | 2019 Convertible Senior Notes |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Net change in fair value of derivatives |  |  |  |  |  |  | $ 162,000,000.0 | $ 221,400,000 |  |

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- Definition The cash inflow from additional borrowings, net of cash paid to third parties in connection with debt origination. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 14 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3255-108585 + Details
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