Condensed Consolidated Statements of Changes in Shareholders' Deficit (Unaudited) - USD ($) $ in Thousands		Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Deficit [Member]	Total
Balance at Dec. 31, 2017	[1]	$ 0	$ (26,495)	$ 0	$ (26,495)
Balance (in shares) at Dec. 31, 2017	[1]	5,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Effect of stock split		$ 15	(15)	0	0
Effect of stock split (in shares)		15,072,647
Net income (loss)		$ 0	0	4,099	4,099
Balance at Mar. 31, 2018		$ 15	(26,510)	4,099	(22,396)
Balance (in shares) at Mar. 31, 2018		15,077,647
Balance at Dec. 31, 2017	[1]	$ 0	(26,495)	0	(26,495)
Balance (in shares) at Dec. 31, 2017	[1]	5,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)					(47,432)
Balance at Sep. 30, 2018		$ 25	70,851	(47,432)	23,444
Balance (in shares) at Sep. 30, 2018		24,942,185
Balance at Mar. 31, 2018		$ 15	(26,510)	4,099	(22,396)
Balance (in shares) at Mar. 31, 2018		15,077,647
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock issued to performance unit plan participants		$ 5	19,929	0	19,934
Common Stock issued to performance unit plan participants (in shares)		4,922,353
Share-based compensation		$ 0	7	0	7
Share-based compensation (in shares)		0
Net income (loss)		$ 0	0	(36,493)	(36,493)
Balance at Jun. 30, 2018		$ 20	(6,574)	(32,394)	(38,948)
Balance (in shares) at Jun. 30, 2018		20,000,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock issued related in initial public offering		$ 5	68,709	0	68,714
Common Stock issued related in initial public offering (in shares)		4,925,727
Issuance costs related to initial public offering		$ 0	(5,230)	0	(5,230)
Issuance costs related to initial public offering (in shares)		0
Reclassification of warrant liability to equity		$ 0	12,951	0	12,951
Share-based compensation		$ 0	995	0	995
Share-based compensation (in shares)		16,458
Net income (loss)		$ 0	0	(15,038)	(15,038)
Balance at Sep. 30, 2018		$ 25	70,851	(47,432)	23,444
Balance (in shares) at Sep. 30, 2018		24,942,185
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adoption of ASU 2014-09 and ASU 2018-07 (Note 3.C.) | ASU 2014-09 and ASU 2018-07 [Member]		$ 0	20	(2,852)	(2,832)
Balance at Dec. 31, 2018		$ 25	71,431	(61,376)	10,080
Balance (in shares) at Dec. 31, 2018		24,957,309
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation		$ 0	1,422	0	1,422
Share-based compensation (in shares)		17,830
Net income (loss)		$ 0	0	(14,726)	(14,726)
Balance at Mar. 31, 2019		$ 25	72,873	(78,954)	(6,056)
Balance (in shares) at Mar. 31, 2019		24,975,139
Balance at Dec. 31, 2018		$ 25	71,431	(61,376)	10,080
Balance (in shares) at Dec. 31, 2018		24,957,309
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)					(53,610)
Balance at Sep. 30, 2019		$ 25	83,354	(117,838)	(34,459)
Balance (in shares) at Sep. 30, 2019		25,042,964
Balance at Mar. 31, 2019		$ 25	72,873	(78,954)	(6,056)
Balance (in shares) at Mar. 31, 2019		24,975,139
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation		$ 0	1,739	0	1,739
Share-based compensation (in shares)		16,128
Shares issued under employee stock purchase plan		$ 0	132	0	132
Shares issued under employee stock purchase plan (in shares)		31,393
Net income (loss)		$ 0	0	(20,472)	(20,472)
Balance at Jun. 30, 2019		$ 25	74,744	(99,426)	(24,657)
Balance (in shares) at Jun. 30, 2019		25,022,660
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation		$ 0	1,810	0	1,810
Share-based compensation (in shares)		20,304
Fair value of warrants issued		$ 0	6,800	0	6,800
Net income (loss)		0	0	(18,412)	(18,412)
Balance at Sep. 30, 2019		$ 25	$ 83,354	$ (117,838)	$ (34,459)
Balance (in shares) at Sep. 30, 2019		25,042,964

[1] Represents balances as of December 31, 2017 as adjusted for the reorganization from LLC to C corporation business structure effective at the close of business on that date.