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v3.19.1

Debt 0.75% Senior convertible notes (Details) | 1 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Nov. 30, 2018 USD ($) d $ / shares shares | Jun. 30, 2017 USD ($) $ / shares | Mar. 31, 2019 USD ($) d | Mar. 31, 2018 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($)
Debt Instrument [Line Items] |  |  |  |  |  |
Non-cash interest expense |  |  | $ 12,200,000 | $ 3,700,000 |  |
Senior Notes | Senior convertible notes due 2022 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Interest rate on convertible notes |  | 0.75% |  |  |  |
Aggregate principal amount |  | $ 400,000,000.0 | 400,000,000.0 |  | $ 400,000,000.0 |
Proceeds from convertible debt |  | $ 389,000,000 |  |  |  |
Initial conversion price of convertible notes (in dollars per share) | $ / shares |  | $ 99.09 |  |  |  |
Contractual coupon interest rate |  |  | 700,000 | 800,000 |  |
Non-cash interest expense |  |  | 3,500,000 | 3,300,000 |  |
Amortization of debt issuance costs |  |  | $ 400,000 | $ 400,000 |  |
Effective interest rate |  |  | 5.14% |  |  |
Conversion price in event of fundamental change |  |  | 100.00% |  |  |
Conversion multiple |  |  | $ 1,000 |  |  |
Conversion percentage based on last reported sale price |  |  | 140.00% |  |  |
Debt instrument, redemption price, percentage |  |  | 100.00% |  |  |
Senior Notes | Senior convertible notes due 2023 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Interest rate on convertible notes | 0.75% |  |  |  |  |
Aggregate principal amount | $ 850,000,000 |  | $ 850,000,000.0 |  | 850,000,000.0 |
Proceeds from convertible debt | $ 836,600,000 |  |  |  |  |
Initial conversion price of convertible notes (in dollars per share) | $ / shares | $ 164.29 |  |  |  |  |
Contractual coupon interest rate |  |  | 1,600,000 |  |  |
Non-cash interest expense |  |  | 7,800,000 |  |  |
Amortization of debt issuance costs |  |  | $ 500,000 |  |  |
Effective interest rate |  |  | 5.63% |  |  |
Conversion price in event of fundamental change |  |  | 100.00% |  |  |
Conversion multiple |  |  | $ 1,000 |  |  |
Threshold consecutive trading days | d | 60 |  |  |  |  |
Conversion percentage based on last reported sale price |  |  | 130.00% |  |  |
Debt instrument, redemption price, percentage |  |  | 100.00% |  |  |
Debt instrument, convertible, conversion ratio | 0.0060869 | 0.0100918 |  |  |  |
Warrants sold to counterparties | shares | 5,200,000 |  |  |  |  |
Proceeds from issuance of warrants | $ 183,800,000 |  |  |  |  |
Strike price of warrant | $ / shares | $ 198.3840 |  |  |  |  |
Senior Notes | Additional Paid-In Capital | Senior convertible notes due 2022 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Amount reclassified to stockholders' equity |  |  |  |  |  | $ 72,600,000
Senior Notes | Additional Paid-In Capital | Senior convertible notes due 2023 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Amount reclassified to stockholders' equity |  |  |  |  | $ 174,400,000 |
Senior Notes | Term of at least 20 trading days during a period of 30 consecutive trading days | Minimum | Senior convertible notes due 2022 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Threshold consecutive trading days | d |  |  | 20 |  |  |
Threshold percentage of stock price trigger |  |  | 130.00% |  |  |
Senior Notes | Term of at least 20 trading days during a period of 30 consecutive trading days | Minimum | Senior convertible notes due 2023 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Threshold consecutive trading days | d |  |  | 20 |  |  |
Threshold percentage of stock price trigger |  |  | 130.00% |  |  |
Senior Notes | Term of at least 20 trading days during a period of 30 consecutive trading days | Maximum | Senior convertible notes due 2022 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Threshold consecutive trading days | d |  |  | 30 |  |  |
Senior Notes | Term of at least 20 trading days during a period of 30 consecutive trading days | Maximum | Senior convertible notes due 2023 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Threshold consecutive trading days | d |  |  | 30 |  |  |
Senior Notes | Term of five business day period after any five consecutive trading day period | Senior convertible notes due 2022 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Threshold consecutive trading days | d |  |  | 5 |  |  |
Senior Notes | Term of five business day period after any five consecutive trading day period | Senior convertible notes due 2023 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Threshold consecutive trading days | d |  |  | 5 |  |  |
Senior Notes | Term of five business day period after any five consecutive trading day period | Maximum | Senior convertible notes due 2022 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Threshold consecutive trading days | d |  |  | 5 |  |  |
Threshold percentage of stock price trigger |  |  | 98.00% |  |  |
Senior Notes | Term of five business day period after any five consecutive trading day period | Maximum | Senior convertible notes due 2023 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Threshold consecutive trading days | d |  |  | 5 |  |  |
Threshold percentage of stock price trigger |  |  | 98.00% |  |  |
Designated as Hedging Instrument | Senior Notes | Senior convertible notes due 2023 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Convertible note hedge, number of shares | shares | 5,173,865 |  |  |  |  |
Cost of convertible note hedge recognized in additional paid-in capital | $ 218,900,000 |  |  |  |  |

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