Prior-Period Financial Statements - Stockholders' Equity (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended
	Mar. 31, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Increase (Decrease) in Stockholders' Equity
Balance				$ 316,464
Balance, revised						$ 316,464
Sale of common stock, net of issuance cost				245,504
Issuance of common stock upon exercise of stock awards				2,915
Issuance of common stock under the Employee Stock Purchase Plan				504
Comprehensive income (loss)		$ (25,676)	$ (7,769)	(55,561)	$ (22,999)
Stock-based compensation expense				9,353
Balance		519,179		519,179		316,464
As Reported Before ASC Topic 606
Increase (Decrease) in Stockholders' Equity
Balance				306,025	337,933	337,933
Issuance of common stock upon exercise of stock awards						2,797
Issuance of common stock under the Employee Stock Purchase Plan						936
Comprehensive income (loss)						(49,292)
Stock-based compensation expense						13,651
Balance						306,025
Effect of Adoption of ASC Topic 606
Increase (Decrease) in Stockholders' Equity
Adoption of ASC 606						10,439
ASU 2014-09
Increase (Decrease) in Stockholders' Equity
Adoption of ASC 606						34,418
ASU 2014-09 | As Reported Before ASC Topic 606
Increase (Decrease) in Stockholders' Equity
Balance				282,046
Adoption of ASC 606							$ 23,979
Comprehensive income (loss)			(6,929)		(21,318)
Balance						282,046
ASU 2014-09 | Effect of Adoption of ASC Topic 606
Increase (Decrease) in Stockholders' Equity
Balance				34,418
Comprehensive income (loss)			$ (840)		(1,681)
Balance						34,418
Common Stock
Increase (Decrease) in Stockholders' Equity
Balance				$ 470
Balance (in shares)				47,002,488
Balance, revised						470
Sale of common stock, net of issuance cost				$ 84
Sale of common stock, net of issuance cost (in shares)	8,395,000			8,395,000
Issuance of common stock upon exercise of stock awards				$ 4
Issuance of common stock upon exercise of stock awards (in shares)				397,270
Issuance of common stock under the Employee Stock Purchase Plan (in shares)				26,552
Balance		$ 558		$ 558		$ 470
Balance (in shares)		55,821,310		55,821,310		47,002,488
Common Stock | As Reported Before ASC Topic 606
Increase (Decrease) in Stockholders' Equity
Balance				$ 470	$ 466	$ 466
Balance (in shares)				47,002,488	46,567,978	46,567,978
Issuance of common stock upon exercise of stock awards						$ 4
Issuance of common stock upon exercise of stock awards (in shares)						363,603
Issuance of common stock under the Employee Stock Purchase Plan (in shares)						70,907
Balance						$ 470
Balance (in shares)						47,002,488
Additional Paid-in Capital
Increase (Decrease) in Stockholders' Equity
Balance				$ 570,670
Balance, revised						$ 570,670
Sale of common stock, net of issuance cost				245,420
Issuance of common stock upon exercise of stock awards				2,911
Issuance of common stock under the Employee Stock Purchase Plan				504
Stock-based compensation expense				9,353
Balance		$ 828,858		828,858		570,670
Additional Paid-in Capital | As Reported Before ASC Topic 606
Increase (Decrease) in Stockholders' Equity
Balance				570,670	$ 553,290	553,290
Issuance of common stock upon exercise of stock awards						2,793
Issuance of common stock under the Employee Stock Purchase Plan						936
Stock-based compensation expense						13,651
Balance						570,670
Additional Paid-in Capital | ASU 2016-09 | As Reported Before ASC Topic 606
Increase (Decrease) in Stockholders' Equity
Adoption of ASC 606							401
Accumulated Other Comprehensive Loss
Increase (Decrease) in Stockholders' Equity
Balance				(1,808)
Balance, revised						(1,808)
Comprehensive income (loss)				(200)
Balance		(2,008)		(2,008)		(1,808)
Accumulated Other Comprehensive Loss | As Reported Before ASC Topic 606
Increase (Decrease) in Stockholders' Equity
Balance				(1,808)	(1,441)	(1,441)
Comprehensive income (loss)						(367)
Balance						(1,808)
Accumulated Deficit
Increase (Decrease) in Stockholders' Equity
Balance				(252,868)
Balance, revised						(252,868)
Comprehensive income (loss)				(55,361)
Balance		$ (308,229)		(308,229)		(252,868)
Accumulated Deficit | As Reported Before ASC Topic 606
Increase (Decrease) in Stockholders' Equity
Balance				(263,307)	(214,382)	(214,382)
Comprehensive income (loss)						(48,925)
Balance						(263,307)
Accumulated Deficit | Effect of Adoption of ASC Topic 606
Increase (Decrease) in Stockholders' Equity
Adoption of ASC 606						10,439
Accumulated Deficit | ASU 2016-09 | As Reported Before ASC Topic 606
Increase (Decrease) in Stockholders' Equity
Adoption of ASC 606							(401)
Accumulated Deficit | ASU 2014-09
Increase (Decrease) in Stockholders' Equity
Adoption of ASC 606						34,418
Accumulated Deficit | ASU 2014-09 | As Reported Before ASC Topic 606
Increase (Decrease) in Stockholders' Equity
Adoption of ASC 606							$ 23,979
Scenario, Previously Reported
Increase (Decrease) in Stockholders' Equity
Balance				282,046
Balance						282,046
Scenario, Previously Reported | As Reported Before ASC Topic 606
Increase (Decrease) in Stockholders' Equity
Balance					313,954	313,954
Scenario, Previously Reported | Common Stock
Increase (Decrease) in Stockholders' Equity
Balance				$ 470
Balance (in shares)				47,002,488
Balance						$ 470
Balance (in shares)						47,002,488
Scenario, Previously Reported | Common Stock | As Reported Before ASC Topic 606
Increase (Decrease) in Stockholders' Equity
Balance					$ 466	$ 466
Balance (in shares)					46,567,978	46,567,978
Scenario, Previously Reported | Additional Paid-in Capital
Increase (Decrease) in Stockholders' Equity
Balance				$ 570,670
Balance						$ 570,670
Scenario, Previously Reported | Additional Paid-in Capital | As Reported Before ASC Topic 606
Increase (Decrease) in Stockholders' Equity
Balance					$ 552,889	552,889
Scenario, Previously Reported | Accumulated Other Comprehensive Loss
Increase (Decrease) in Stockholders' Equity
Balance				(1,808)
Balance						(1,808)
Scenario, Previously Reported | Accumulated Other Comprehensive Loss | As Reported Before ASC Topic 606
Increase (Decrease) in Stockholders' Equity
Balance					(1,441)	(1,441)
Scenario, Previously Reported | Accumulated Deficit
Increase (Decrease) in Stockholders' Equity
Balance				$ (287,286)
Balance						(287,286)
Scenario, Previously Reported | Accumulated Deficit | As Reported Before ASC Topic 606
Increase (Decrease) in Stockholders' Equity
Balance					$ (237,960)	$ (237,960)