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Debt (Details)	1 Months Ended		3 Months Ended	12 Months Ended
	Feb. 28, 2018 USD ($)	Jan. 31, 2018 USD ($) day $ / shares	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Debt
Underwriting discounts and commissions and other offering expenses				$ 8,440,000
Proceeds from issuance of 1.75% convertible senior notes due 2025				450,000,000	$ 0	$ 0
Interest expense				25,472,000	5,925,000	3,498,000
Repayments of debt				55,000,000	0	0
Loss on extinguishment of debt			$ 2,200,000	2,209,000	$ 0	$ 0
1.75% Convertible Senior Note Due 2025
Debt
Initial conversion rate		0.0255384
A&R Loan Agreement
Debt
Repayments of debt	$ 55,000,000.0
Repayments of debt, back end fees, outstanding interest, and prepayment penalty fees	$ 3,200,000
Convertible Notes Payable
Debt
Aggregate principal amount				450,000,000.0
Convertible Notes Payable | 1.75% Convertible Senior Note Due 2025
Debt
Aggregate principal amount		$ 450,000,000.0
Option to purchase additional debt		50,000,000.0
Underwriting discounts and commissions and other offering expenses		14,200,000
Proceeds from issuance of 1.75% convertible senior notes due 2025		$ 435,800,000
Initial conversion price (in dollars per share) | $ / shares		$ 39.16
Convertible notes ($450.0 face value)		$ 309,100,000
Effective interest rate		7.60%
Equity component of convertible debt		$ 140,900,000
Remaining discount amortization period		6 years 14 days
Interest expense				$ 25,500,000
Conversion Term (i) | Convertible Notes Payable | 1.75% Convertible Senior Note Due 2025
Debt
Threshold trading days | day		5
Threshold consecutive trading days | day		5
Threshold percentage of stock price trigger		98.00%
Conversion Term (ii) | Convertible Notes Payable | 1.75% Convertible Senior Note Due 2025
Debt
Threshold trading days | day		45
Threshold consecutive trading days | day		10
Threshold percentage of stock price trigger		10.00%
Conversion Term (iii) | Convertible Notes Payable | 1.75% Convertible Senior Note Due 2025
Debt
Threshold trading days following fundamental change | day		30
Conversion Term (iv) | Convertible Notes Payable | 1.75% Convertible Senior Note Due 2025
Debt
Threshold trading days | day		20
Threshold consecutive trading days | day		30
Threshold percentage of stock price trigger		130.00%

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