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Basis of Presentation and Summary of Significant Accounting Policies - Derivatives (Details) - Convertible Notes - USD ($) | Jun. 30, 2018 | Jun. 19, 2018 | May 22, 2018 | Sep. 30, 2018 | Dec. 31, 2017 | Aug. 31, 2014
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2023 Convertible Senior Notes |  |  |  |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |  |  |  |
Aggregate principal amount outstanding |  |  | $ 750,000,000 | $ 750,000,000 | $ 0 |
Interest rate (as a percent) |  |  | 0.75% |  |  |
Common stock price (USD per share) |  | $ 195.91 | $ 187.09 |  |  |
Exercise price, conversion premium and bond hedge (USD per share) |  | 248.48 | 248.48 |  |  |
Exercise price, warrant (USD per share) |  | $ 315.38 | $ 315.38 |  |  |
Risk-free interest rate (as a percent) |  | 2.80% | 2.90% |  |  |
Dividend yield (as a percent) |  | 0.00% | 0.00% |  |  |
Annual coupon rate (as a percent) |  | 0.75% | 0.75% |  |  |
Remaining contractual term (in years) |  | 4 years 11 months 23 days | 5 years |  |  |
2023 Convertible Senior Notes | Minimum |  |  |  |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |  |  |  |
Volatility (as a percent) |  | 30.00% | 30.00% |  |  |
2023 Convertible Senior Notes | Maximum |  |  |  |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |  |  |  |
Volatility (as a percent) |  | 35.00% | 35.00% |  |  |
2019 Convertible Senior Notes |  |  |  |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |  |  |  |
Aggregate principal amount outstanding |  |  |  | $ 223,215,000 | $ 245,000,000 | $ 245,000,000.0
Interest rate (as a percent) |  |  |  |  |  | 0.75%
Common stock price (USD per share) |  |  | $ 187.09 | $ 274.49 |  |
Exercise price, conversion premium and bond hedge (USD per share) |  |  | $ 75.05 | $ 75.05 |  | $ 75.05
Risk-free interest rate (as a percent) | 2.59% |  | 2.47% |  |  |
Dividend yield (as a percent) | 0.00% |  | 0.00% |  |  |
Annual coupon rate (as a percent) | 0.75% |  | 0.75% |  |  |
Remaining contractual term (in years) | 10 months 20 days |  | 1 year 3 months |  |  |
2019 Convertible Senior Notes | Minimum |  |  |  |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |  |  |  |
Volatility (as a percent) | 30.00% |  | 30.00% |  |  |
2019 Convertible Senior Notes | Maximum |  |  |  |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |  |  |  |
Volatility (as a percent) | 35.00% |  | 35.00% |  |  |

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