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Convertible Senior Notes - Narrative (Details) |  |  | 1 Months Ended | 2 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | ---
Dec. 31, 2018 USD ($) $ / shares shares | May 22, 2018 USD ($) $ / shares shares | Nov. 30, 2018 USD ($) shares | Jun. 19, 2018 USD ($) $ / shares | May 31, 2018 trading_day | Aug. 31, 2014 USD ($) day trading_day $ / shares shares | Nov. 30, 2018 USD ($) | Aug. 31, 2018 USD ($) | Apr. 30, 2018 USD ($) | Dec. 31, 2018 USD ($) $ / shares shares | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Carrying value of equity component of convertible notes | $ 0 |  |  |  |  |  |  |  |  | $ 0 | $ 18,859,000 |
Warrants issued in public offering (shares) | shares |  |  |  |  |  | 3,264,643 |  |  |  |  |  |
Exercise price of convertible bond hedge (USD per share) | $ / shares |  |  |  |  |  | $ 75.05 |  |  |  |  |  |
Derivative asset | 22,576,000 |  |  |  |  |  |  |  |  | 22,576,000 | 0 |
Net change in fair value of derivatives |  |  |  |  |  |  |  |  |  | 1,800,000 |  |
Warrant exercise price (USD per share) | $ / shares |  |  |  |  |  | $ 125.08 |  |  |  |  |  |
Value of warrants issued |  |  |  |  |  | $ 11,600,000 |  |  |  |  |  |
Debt issuance costs related to the equity component of convertible debt |  |  |  |  |  |  |  |  |  | 18,859,000 | 10,704,000 | $ 10,065,000
Convertible debt, noncurrent | $ 609,864,000 |  |  |  |  |  |  |  |  | 609,864,000 | 0 |
Warrants settled (in shares) | shares |  |  | 525,000 |  |  |  |  |  |  |  |  |
Deduction to additional paid-in-capital for warrants settled |  |  |  |  |  |  |  |  |  | 28,300,000 |  |
Payments for repurchase of warrants |  |  | $ 30,100,000 |  |  |  |  |  |  | 30,094,000 | 0 | $ 0
Gain (loss) on derivative liabilities |  |  |  |  |  |  |  |  |  | $ (1,800,000) |  |
Warrants outstanding (in shares) | shares | 2,739,643 |  |  |  |  |  |  |  |  | 2,739,643 |  |
2023 Convertible Senior Notes |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Value of warrants issued |  |  |  |  |  |  |  |  |  | $ 97,805,000 |  |
Convertible Notes | 2019 Convertible Senior Notes |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Principal amount outstanding | $ 27,326,000 |  |  |  |  | 245,000,000.0 |  |  |  | 27,326,000 | 245,000,000 |
Net proceeds from debt issuance |  |  |  |  |  | $ 239,300,000 |  |  |  |  |  |
Effective rate (as a percent) |  |  |  |  |  | 5.83% |  |  |  |  |  |
Convertible debt conversion ratio |  |  |  |  |  | 0.0133251 |  |  |  |  |  |
Debt conversion price per share (USD per share) | $ / shares |  |  |  |  |  | $ 75.05 |  |  |  |  |  |
Interest on debt instrument (as a percent) |  |  |  |  |  | 0.75% |  |  |  |  |  |
If-converted value in excess of principal | $ 23,400,000 | $ 341,600,000 |  |  |  |  |  |  |  |  |  |
Increase (decrease) in if-converted value in excess of principal |  |  |  |  |  |  |  |  |  | 118,700,000 |  |
Notices for conversion |  |  |  |  |  |  |  | $ 195,900,000 | $ 21,800,000 |  |  |
Conversion value over the principal amount |  |  |  |  |  |  | $ 439,600,000 |  | $ 31,600,000 |  |  |
Gain (loss) on extinguishment of debt |  |  |  |  |  |  |  |  |  | $ (3,200,000) |  |
Exercise price of convertible bond hedge (USD per share) | $ / shares | $ 75.05 | $ 75.05 |  |  |  |  |  |  |  | $ 75.05 |  |
Payments for convertible bond hedges |  |  |  |  |  | $ 48,100,000 |  |  |  |  |  |
Derivative asset | $ 22,600,000 | $ 340,000,000.0 |  |  |  |  |  |  |  | $ 22,600,000 |  |
Net change in fair value of derivatives |  |  |  |  |  |  |  |  |  | (119,400,000) |  |
Proceeds from derivative instrument |  |  |  |  |  |  |  |  |  | 471,200,000 |  |
Gain (loss) on derivative liabilities |  |  |  |  |  |  |  |  |  | 119,400,000 |  |
Convertible Notes | 2019 Convertible Senior Notes | Redemption Period One |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Number of trading days | trading_day |  |  |  |  |  | 20 |  |  |  |  |  |
Number of consecutive trading days | trading_day |  |  |  |  |  | 30 |  |  |  |  |  |
Stock price trigger to classify convertible debt as current (as a percent) |  |  |  |  |  | 130.00% |  |  |  |  |  |
Convertible Notes | 2019 Convertible Senior Notes | Redemption Period Two |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Number of trading days | day |  |  |  |  |  | 5 |  |  |  |  |  |
Number of consecutive trading days | trading_day |  |  |  |  |  | 10 |  |  |  |  |  |
Maximum threshold of debt trading price trigger (as a percent) |  |  |  |  |  | 98.00% |  |  |  |  |  |
Convertible Notes | 2023 Convertible Senior Notes |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Principal amount outstanding | 750,000,000 | 750,000,000 |  |  |  |  |  |  |  | 750,000,000 | 0 |
Net proceeds from debt issuance |  | $ 733,100,000 |  |  |  |  |  |  |  |  |  |
Convertible debt conversion ratio |  |  |  |  | 0.0040244 |  |  |  |  |  |  |
Debt conversion price per share (USD per share) | $ / shares |  | $ 248.48 |  |  |  |  |  |  |  |  |  |
Interest on debt instrument (as a percent) |  | 0.75% |  |  |  |  |  |  |  |  |  |
Carrying value of equity component of convertible notes | $ 127,997,000 |  |  |  |  |  |  |  |  | 127,997,000 | $ 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 the conversion option derivative liability |  | 144,000,000.0 |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |
Increase (decrease) in derivative assets |  |  |  |  |  |  |  |  |  | 19,200,000 |  |
Increase (decrease) in derivative liabilities |  |  |  | $ 13,500,000 |  |  |  |  |  | $ 7,500,000 |  |
Convertible Notes | 2023 Convertible Senior Notes | Redemption Period One |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Number of trading days | trading_day |  |  |  |  | 20 |  |  |  |  |  |  |
Number of consecutive trading days | trading_day |  |  |  |  | 30 |  |  |  |  |  |  |
Stock price trigger to classify convertible debt as current (as a percent) |  |  |  |  | 130.00% |  |  |  |  |  |  |
Convertible Notes | 2023 Convertible Senior Notes | Redemption Period Two |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Number of trading days | trading_day |  |  |  |  | 5 |  |  |  |  |  |  |
Number of consecutive trading days | trading_day |  |  |  |  | 10 |  |  |  |  |  |  |
Maximum threshold of debt trading price trigger (as a percent) |  |  |  |  | 98.00% |  |  |  |  |  |  |

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