Income Taxes - Expense (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Jun. 30, 2016
Reconciliation of the expected statutory tax benefit to the actual income taxes
U.S. federal corporate tax rate (as a percent)	34.00%	21.00%	34.00%	34.00%
Loss before income tax expense	$ (78,883)	$ (168,843)	$ (96,012)	$ (144,817)
Expected tax benefit at 21%, 34%, 34% and 34%, respectively	(26,820)	(35,457)	(32,644)	(49,238)
Permanent differences	15	(103)	25	345
Incentive stock options	1,313	1,144	1,528	2,501
State tax benefit net of federal benefit	(4,157)	(10,622)	(3,537)	(7,954)
Change in valuation allowance, net	32,922	53,706	(63,238)	62,505
Federal research credit	(1,232)	(2,466)	(2,204)	(4,109)
Federal orphan drug credit	(2,901)	(6,934)	(7,118)	(4,241)
Expired loss and credit carryforwards				184
Change in U.S. tax law			97,479
Debt inducement			8,044
Lease incentive		109
Stock option expirations	860	623	1,665	7
Benefit for income taxes