Stockholders’ Equity (Details) $ / shares in Units, $ in Thousands										6 Months Ended	12 Months Ended
	Dec. 09, 2019 USD ($) $ / shares shares	Mar. 29, 2019 USD ($) $ / shares	Jan. 01, 2019 shares	Jun. 15, 2018 USD ($) $ / shares shares	Dec. 05, 2017 USD ($) shares	Jul. 31, 2017 shares	May 10, 2017 USD ($) $ / shares shares	Feb. 10, 2017 USD ($) $ / shares shares	Oct. 11, 2016 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares	Jun. 30, 2018 USD ($) shares	Jun. 30, 2017 USD ($)	Jun. 29, 2017 shares	Jun. 28, 2017 shares	Feb. 16, 2017 USD ($)
Class of Warrant or Right [Line Items]
Common and preferred shares authorized (in shares)														260,000,000	165,000,000
Common stock, shares authorized (in shares)										250,000,000	250,000,000			250,000,000	155,000,000
Preferred stock, shares authorized (in shares)											10,000,000			10,000,000	10,000,000
Preferred stock, par value (in USD per share) | $ / shares											$ 0.01
Agreement purchase shares of common stock (in shares)			4,400,000
Common stock, par value (in USD per share) | $ / shares										$ 0.01	$ 0.01
Exercise of warrants | $										$ 1,688	$ 0	$ 78,226	$ 0
Number of shares issued in transaction (in shares)	16,428,572			13,225,000
Proceeds from public offering of common stock | $										$ 0	273,031	299,467	28,578
Payments for stock issuance costs | $												17,400
Consideration received per transaction | $											$ 71,600
Retired treasury stock (in shares)										105,959
Convertible preferred stock
Class of Warrant or Right [Line Items]
Preferred stock, shares authorized (in shares)										10,000,000	10,000,000
Preferred stock, par value (in USD per share) | $ / shares										$ 0.01	$ 0.01
Preferred stock, shares issued (in shares)										0	0
Seattle Genetics, Inc., (“SGEN”)
Class of Warrant or Right [Line Items]
Exercise of warrants (in shares)					8,655,804
Aggregate purchase price of common stock | $					$ 42,400
Securities Purchase Agreement | Seattle Genetics, Inc., (“SGEN”)
Class of Warrant or Right [Line Items]
Agreement purchase shares of common stock (in shares)						3,000,000		3,000,000
Common stock, par value (in USD per share) | $ / shares								$ 4.90
Gross proceeds from sale of common stock | $								$ 14,700
Warrants term								3 years
Shares that warrants can purchase (in shares)						8,655,804		8,655,804
Fair value of warrants | $																$ 22,300
Private placement
Class of Warrant or Right [Line Items]
Net proceeds from sale of common stock | $										$ 0	$ 0	67,784	$ 14,700
Private placement | Convertible preferred stock
Class of Warrant or Right [Line Items]
Preferred stock, shares issued (in shares)							1,000,000
Issuance of convertible preferred stock (in USD per share) | $ / shares							$ 125
Proceeds from issuance of convertible preferred stock | $							$ 125,000
Convertible preferred stock, shares issued upon conversion (in shares)							23.10536
Conversion of shares (in shares)						23,105,348	23,105,348
Share price (in USD per share) | $ / shares							$ 5.41
IPO
Class of Warrant or Right [Line Items]
Common stock, par value (in USD per share) | $ / shares									$ 3.00
Gross proceeds from sale of common stock | $									$ 30,000
Shares that warrants can purchase (in shares)									10,000,000
Exercise of warrants (in shares)										450,000
Net proceeds from sale of common stock | $									$ 28,600
Exercise price of warrant (in USD per share) | $ / shares									$ 3.75
Increase in fair value of warrants exercised | $												$ 102,100
Warrants outstanding (in shares)										0	450,000
Exercise of warrants | $										$ 1,700
Number of shares issued in transaction (in shares)	14,285,715			11,500,000
Proceeds from public offering of common stock | $	$ 250,000
Sale of stock (in USD per share) | $ / shares	$ 17.50			$ 24.00
IPO | Securities Purchase Agreement
Class of Warrant or Right [Line Items]
Fair value of warrants | $									$ 7,300
Over-Allotment Option [Member]
Class of Warrant or Right [Line Items]
Number of shares issued in transaction (in shares)	2,142,857			1,725,000
Proceeds from initial public offering, gross | $	$ 287,500
Proceeds from public offering of common stock | $	$ 273,000			$ 299,500
ATM Agreement
Class of Warrant or Right [Line Items]
Common stock, par value (in USD per share) | $ / shares		$ 0.01
Sale of stock, consideration received on transaction | $		$ 150,000
Payments for stock issuance costs percentage (as percent)		3.00%
Number of shares issued in transaction (in shares)											4,432,416
Sales Agreement
Class of Warrant or Right [Line Items]
Treasury stock, acquired (in shares)											84,896
Warrant | IPO
Class of Warrant or Right [Line Items]
Warrants exercised (in shares)												9,550,000
Weighted Average
Class of Warrant or Right [Line Items]
Shares issued, price (USD per share) | $ / shares											$ 16.40