Shareholders' Equity - Narrative (Details) - USD ($)	May 24, 2019	Sep. 30, 2019	Dec. 31, 2018	Jan. 31, 2018
Class of Stock [Line Items]
Common stock, authorized shares (in shares)		500,000,000	500,000,000
Common stock, par value (in dollars per share)		$ 0.01	$ 0.01
Common stock, issued shares (in shares)		89,310,684	77,307,521
Common stock, outstanding shares (in shares)		89,310,684	77,307,521
Preferred stock, authorized (in shares)		200,000,000
Preferred stock, par value (in dollars per share)		$ 0.01
Preferred stock, shares issued (in shares)		0
Preferred stock, shares outstanding (in shares)		0
Convertible debt securities
Class of Stock [Line Items]
Common stock shares reserved for issuance (in shares)		11,492,280
Stock options to purchase common stock
Class of Stock [Line Items]
Common stock shares reserved for issuance (in shares)		10,773,384
Unvested RSUs
Class of Stock [Line Items]
Common stock shares reserved for issuance (in shares)		459,747
1.75 convertible senior notes due 2025 | Convertible debt securities
Class of Stock [Line Items]
Aggregate principal amount				$ 450,000,000.0
Convertible Notes ($450.0 face value)				309,100,000
Equity component of convertible debt				$ 140,900,000
IPO
Class of Stock [Line Items]
Shares issued in period (in shares)	10,657,692
Stock options exercised (in dollars per share)	$ 26.00
Underwriters' discount and offering costs	$ 16,000,000.0
Proceeds from issuance IPO	$ 261,100,000
Over-Allotment Option
Class of Stock [Line Items]
Stock options exercised (in shares)	1,042,307
Chief Executive Officer
Class of Stock [Line Items]
Stock options exercised (in shares)	400,000