SUPPLEMENTARY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
SUPPLEMENTARY FINANCIAL DATA (UNAUDITED)
Revenue	$ 15,737	$ 12,838	$ 23,476	$ 8,319	$ 4,515	$ 4,275	$ 3,509	$ 3,087	$ 60,370	$ 15,386	$ 48,648
Costs and expenses	78,358	75,288	72,180	73,295	83,691	61,272	68,630	61,916	299,121	275,509	229,115
Loss from operations	(62,621)	(62,450)	(48,704)	(64,976)	(79,176)	(56,997)	(65,121)	(58,829)	(238,751)	(260,123)	(180,467)
Net loss	$ (50,186)	$ (59,433)	$ (40,818)	$ (65,087)	$ (86,922)	$ (66,877)	$ (66,287)	$ (65,319)	$ (215,524)	$ (285,405)	$ (190,669)
Basic and diluted net loss per share (in dollars per share)	$ (0.92)	$ (1.10)	$ (0.76)	$ (1.22)	$ (1.64)	$ (1.27)	$ (1.27)	$ (1.27)	$ (3.99)	$ (5.45)	$ (4.26)