Net Loss per Share Attributable to Common Stockholders (Computation of Basic and Diluted Net Loss per Share Attributable to Common Stockholders) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended									12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016		Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net Loss per Share Attributable to Common Stockholders [Abstract]
Net loss available to common stockholders	$ (18,680)	$ (22,193)	$ (22,783)	$ (18,036)	$ (21,853)	$ (138,428)	[1]	$ (37,034)	$ (33,173)	$ (81,692)	$ (230,488)	$ (147,082)
Weighted average common shares outstanding used to compute basic and diluted net loss per share	78,397,156	52,572,880	51,846,521	46,507,435	45,078,666	32,678,259	[2]	8,304,334	8,302,635	57,405,331	23,674,512	8,127,781
Net loss per share, basic and diluted	$ (0.24)	$ (0.42)	$ (0.44)	$ (0.39)	$ (0.48)	$ (4.24)		$ (4.46)	$ (4.00)	$ (1.42)	$ (9.74)	$ (18.10)

[1] Net loss attributable to common stockholders for the three months ended September 30, 2016 includes the beneficial conversion feature of the Company's debt upon conversion into shares of the Company's common stock on August 1, 2016 of $44.2 million, the beneficial conversion feature of certain outstanding warrants which became exercisable into shares of the Company's common stock on August 1, 2016 of $1.7 million, the deemed dividends on the Company's convertible preferred stock and Class E redeemable convertible units of $20.9 million and share-based compensation expense related to the Company's LTIP of $22.6 million.

[2] Weighted average basic and diluted shares of common stock outstanding for the three months ended September 30, 2016 includes shares issued as a result of the Corporate Conversion (Note 1).