Convertible Senior Notes and Capped Call Transactions (Narrative) (Details) $ / shares in Units, shares in Thousands	1 Months Ended	3 Months Ended
	Mar. 31, 2019 USD ($) $ / shares	Mar. 31, 2019 USD ($) equity_instrument day $ / shares shares	Dec. 31, 2018 $ / shares
Debt Instrument [Line Items]
Common stock, par value (in dollars per share) | $ / shares	$ 0.001	$ 0.001	$ 0.001
Carry amount of equity component of convertible notes	$ 52,900,000	$ 52,900,000
Total deferred issuance costs	8,600,000	8,600,000
Debt issuance costs attributable to liability	$ 6,600,000	$ 6,600,000
Remaining amortization period for debt discount and debt issuance costs		4 years 10 months 24 days
Effective interest rate (percent)		6.97%
Number of shares of common stock covered by called caps (shares) | shares		5,960
Reduction to additional paid-in-capital related to called caps		$ 23,184,000
Adjustments to Additional Paid in Capital, Debt Conversion Option, Issuance Costs		$ 1,986,000
Capped Calls
Debt Instrument [Line Items]
Capped caps initial strike price (in dollars per share) | $ / shares	$ 38.58	$ 38.58
Capped caps initial cap price (in dollars per share) | $ / shares	$ 57.16	$ 57.16
Convertible Debt | Convertible Senior Notes
Debt Instrument [Line Items]
Aggregate principal	$ 230,000,000	$ 230,000,000
Debt instrument stated rate (percent)	0.75%	0.75%
Aggregate principal from exercise in full of over-allotment options	$ 30,000,000	$ 30,000,000
Proceeds from debt offering, net of debt issuance costs	221,000,000
Debt instrument, unit of principal for conversion	$ 1,000	$ 1,000
Number of shares per convertible note | equity_instrument		25.9182
Common stock, par value (in dollars per share) | $ / shares	$ 0.001	$ 0.001
Convertible debt conversion price (in dollars per share) | $ / shares	$ 38.58	$ 38.58
Percentage of principal amount paid if repurchase due to fundamental change (percent)		100.00%
Threshold trading days in consideration of note conversion | day		20
Threshold consecutive trading days in analysis of conversion price | day		30
Threshold percentage of stock price if converted		130.00%
Threshold for five day period, product of sale price of common stock and conversion rate of notes		98.00%
Carry amount of equity component of convertible notes	$ 52,900,000	$ 52,900,000
Additional Paid-in Capital
Debt Instrument [Line Items]
Reduction to additional paid-in-capital related to called caps		23,184,000
Adjustments to Additional Paid in Capital, Debt Conversion Option, Issuance Costs		$ 1,986,000