Summary of Significant Accounting Policies - Revenue (Details) $ in Thousands	1 Months Ended					3 Months Ended					6 Months Ended		12 Months Ended
	Feb. 28, 2018 item	Dec. 31, 2015 item	Mar. 31, 2015 item	May 31, 2013 item	Mar. 31, 2013 item	Mar. 31, 2018 USD ($)	Dec. 31, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2017 USD ($) item	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Dec. 31, 2011 item	Dec. 31, 2010 item	Dec. 31, 2000 item
Summary of Significant Accounting Policies
Number of types of licensing and development agreements with collaborative partners													3
Proceeds from common stock issuance, net of $222 of transaction costs | $													$ 101,663
Royalty revenue | $							$ 195	$ 5,099	$ 4,625	$ 4,166		$ 195		$ 195	$ 13,867
Difference between full cost and amounts received | $											$ 900		$ 3,100	$ 6,900	$ 9,200
Number of types of milestone payments under collaborative arrangements													3
Number of quarters in arrear for revenue recognition													1
Minimum
Summary of Significant Accounting Policies
Period to earn royalty payments													10 years
Maximum
Summary of Significant Accounting Policies
Period to earn royalty payments													12 years
Kadcyla | Minimum
Summary of Significant Accounting Policies
Period to earn royalty payments													10 years
Kadcyla | Maximum
Summary of Significant Accounting Policies
Period to earn royalty payments													12 years
Forecast
Summary of Significant Accounting Policies
Reduction in current workforce, number	20
Severance charges | $						$ 2,500
Charge for retention benefits | $						$ 2,500
Amgen
Summary of Significant Accounting Policies
Number of single-target licenses		4		1									2					3
Bayer
Summary of Significant Accounting Policies
Number of single-target licenses													1
Biotest
Summary of Significant Accounting Policies
Number of single-target licenses													1
CytomX
Summary of Significant Accounting Policies
Number of single-target licenses													1
Fusion Pharmaceuticals
Summary of Significant Accounting Policies
Number of single-target licenses													1
Lilly
Summary of Significant Accounting Policies
Number of single-target licenses													3			3
Novartis
Summary of Significant Accounting Policies
Number of single-target licenses					1								5				6
Number of licenses to two related targets													1
Number of related targets					2								2
Number of related targets licenses on exclusive basis													1
Roche
Summary of Significant Accounting Policies
Number of single-target licenses													5
Roche | Undisclosed Target
Summary of Significant Accounting Policies
Number of related targets																		4
Sanofi
Summary of Significant Accounting Policies
Number of single-target licenses													5
Takeda
Summary of Significant Accounting Policies
Number of single-target licenses			2										1
Oxford BioTherapeutics Ltd Member
Summary of Significant Accounting Policies
Number of single-target licenses													1
Debiopharm
Summary of Significant Accounting Policies
Number of single-target licenses													1