Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Unrealized Gain/(Loss)	Accumulated Deficit
Balance at Dec. 31, 2017	$ 122,574		$ 493,823	$ (442)	$ (370,807)
Balance, shares at Dec. 31, 2017		47,612,619
Issuance of common stock, net of issuance costs	95,416		95,416
Issuance of common stock, net of issuance costs, shares		9,194,306
Proceeds from sale of stock under employee stock purchase plan	218		218
Proceeds from sale of stock under employee stock purchase plan, shares		17,569
Exercise of options	182		182
Exercise of options, shares		21,363
Share-based compensation expense	2,232		2,232
Restricted stock unit vesting, shares		11,613
Unrealized gain/loss	(108)			(108)
Net income (loss)	(23,418)				(23,418)
Balance at Mar. 31, 2018	197,096		591,871	(550)	(394,225)
Balance, shares at Mar. 31, 2018		56,857,470
Proceeds from sale of stock under employee stock purchase plan	(3)		(3)
Exercise of options	314		314
Exercise of options, shares		43,041
Share-based compensation expense	2,494		2,494
Restricted stock unit vesting, shares		13,375
Unrealized gain/loss	91			91
Net income (loss)	(34,069)				(34,069)
Balance at Jun. 30, 2018	165,923		594,676	(459)	(428,294)
Balance, shares at Jun. 30, 2018		56,913,886
Issuance of common stock, net of issuance costs	36		36
Proceeds from sale of stock under employee stock purchase plan	267		267
Proceeds from sale of stock under employee stock purchase plan, shares		31,199
Exercise of options	59		59
Exercise of options, shares		101,478
Share-based compensation expense	2,252		2,252
Unrealized gain/loss	50			50
Net income (loss)	(26,047)				(26,047)
Balance at Sep. 30, 2018	142,540		597,290	(409)	(454,341)
Balance, shares at Sep. 30, 2018		57,046,563
Keryx Merger	527,754	$ 1	527,753
Keryx Merger, shares		57,773,090
Issuance of Baupost Additional Shares	13,386		13,386
Issuance of Baupost Additional Shares, shares		1,497,320
Exercise of options	92		92
Exercise of options, shares		12,500
Share-based compensation expense	12,062		12,062
Restricted stock unit vesting, shares		558,045
Unrealized gain/loss	148			148
Net income (loss)	(60,054)				(60,054)
Balance at Dec. 31, 2018	$ 635,928	$ 1	1,150,583	(261)	(514,395)
Balance, shares at Dec. 31, 2018	116,887,518	116,887,518
Proceeds from sale of stock under employee stock purchase plan	$ 188		188
Proceeds from sale of stock under employee stock purchase plan, shares		39,977
Exercise of options	365		365
Exercise of options, shares		62,204
Share-based compensation expense	2,094		2,094
Restricted stock unit vesting, shares		132,563
Unrealized gain/loss	225			225
Net income (loss)	(72,421)				(72,421)
Balance at Mar. 31, 2019	$ 566,379	$ 1	$ 1,153,230	$ (36)	$ (586,816)
Balance, shares at Mar. 31, 2019	117,122,262	117,122,262