Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Shares	Capital in Excess of Par Value	Accumulated Other Comprehensive Loss	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2017		159,524,000
Beginning balance at Dec. 31, 2017	$ 87,828	$ 15	$ 2,207,865	$ (2,111)	$ (2,117,941)
Shares issued under equity compensation plans (in shares)		2,855,000
Shares issued under equity compensation plans	34,406	$ 1	34,405
Stock-based compensation	19,949		19,949
Adoption of new accounting standards	12,577				12,577
Other comprehensive loss	(685)			(685)
Net loss	(95,792)				(95,792)
Ending balance (in shares) at Mar. 31, 2018		162,379,000
Ending balance at Mar. 31, 2018	58,283	$ 16	2,262,219	(2,796)	(2,201,156)
Beginning balance (in shares) at Dec. 31, 2017		159,524,000
Beginning balance at Dec. 31, 2017	$ 87,828	$ 15	2,207,865	(2,111)	(2,117,941)
Shares issued under equity compensation plans (in shares)	4,605,724
Net loss	$ 875,668
Ending balance (in shares) at Jun. 30, 2018		172,415,000
Ending balance at Jun. 30, 2018	1,860,414	$ 17	3,094,095	(4,002)	(1,229,696)
Beginning balance (in shares) at Mar. 31, 2018		162,379,000
Beginning balance at Mar. 31, 2018	$ 58,283	$ 16	2,262,219	(2,796)	(2,201,156)
Shares issued under equity compensation plans (in shares)	1,750,908	1,751,000
Shares issued under equity compensation plans	$ 20,987		20,987
Stock-based compensation	20,659		20,659
Other comprehensive loss	(1,206)			(1,206)
Sale of stock to Bristol-Myers Squibb (in shares)		8,285,000
Sale of stock to Bristol-Myers Squibb	790,231	$ 1	790,230	0	0
Net loss	971,460				971,460
Ending balance (in shares) at Jun. 30, 2018		172,415,000
Ending balance at Jun. 30, 2018	1,860,414	$ 17	3,094,095	(4,002)	(1,229,696)
Beginning balance (in shares) at Dec. 31, 2018		173,530,000
Beginning balance at Dec. 31, 2018	1,717,575	$ 17	3,147,925	(6,316)	(1,424,051)
Shares issued under equity compensation plans (in shares)		698,000
Shares issued under equity compensation plans	5,463		5,463
Stock-based compensation	25,385		25,385
Other comprehensive loss	3,600			3,600
Net loss	(118,512)				(118,512)
Ending balance (in shares) at Mar. 31, 2019		174,228,000
Ending balance at Mar. 31, 2019	1,633,511	$ 17	3,178,773	(2,716)	(1,542,563)
Beginning balance (in shares) at Dec. 31, 2018		173,530,000
Beginning balance at Dec. 31, 2018	$ 1,717,575	$ 17	3,147,925	(6,316)	(1,424,051)
Shares issued under equity compensation plans (in shares)	1,436,538
Net loss	$ (228,421)
Ending balance (in shares) at Jun. 30, 2019		174,966,000
Ending balance at Jun. 30, 2019	1,557,155	$ 17	3,210,398	(788)	(1,652,472)
Beginning balance (in shares) at Mar. 31, 2019		174,228,000
Beginning balance at Mar. 31, 2019	$ 1,633,511	$ 17	3,178,773	(2,716)	(1,542,563)
Shares issued under equity compensation plans (in shares)	738,538	738,000
Shares issued under equity compensation plans	$ 7,103		7,103
Stock-based compensation	24,522		24,522
Other comprehensive loss	1,928			1,928
Net loss	(109,909)				(109,909)
Ending balance (in shares) at Jun. 30, 2019		174,966,000
Ending balance at Jun. 30, 2019	$ 1,557,155	$ 17	$ 3,210,398	$ (788)	$ (1,652,472)