Condensed Statements of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Cumulative Effect, Period of Adoption, Adjustment	Cumulative Effect, Period of Adoption, Adjusted Balance	Common Stock	Common Stock Cumulative Effect, Period of Adoption, Adjusted Balance	Additional Paid-In Capital	Additional Paid-In Capital Cumulative Effect, Period of Adoption, Adjustment	Additional Paid-In Capital Cumulative Effect, Period of Adoption, Adjusted Balance	Accumulated Deficit	Accumulated Deficit Cumulative Effect, Period of Adoption, Adjustment	Accumulated Deficit Cumulative Effect, Period of Adoption, Adjusted Balance	Accumulated Other Comprehensive Income (Loss)	Accumulated Other Comprehensive Income (Loss) Cumulative Effect, Period of Adoption, Adjusted Balance	Treasury Stock	Treasury Stock Cumulative Effect, Period of Adoption, Adjusted Balance
Beginning balance (in shares) at Dec. 31, 2019				27,497,911
Beginning balance at Dec. 31, 2019	$ 19,950			$ 27		$ 715,166			$ (695,266)			$ 23		$ 0
Increase (Decrease) in Stockholders' Equity
Exercise of stock options (in shares)				40,133
Exercise of stock options	1,014			$ 1		1,013
Vesting of restricted stock units (in shares)				10,089
Vesting of restricted stock units	0
Stock-based compensation	7,053					7,053
Other comprehensive loss	(14)											(14)
Net income (loss)	(78,249)								(78,249)
Ending balance (in shares) at Mar. 31, 2020				27,548,133
Ending balance at Mar. 31, 2020	(50,246)			$ 28		723,232			(773,515)			9		0
Beginning balance (in shares) at Dec. 31, 2019				27,497,911
Beginning balance at Dec. 31, 2019	19,950			$ 27		715,166			(695,266)			23		0
Increase (Decrease) in Stockholders' Equity
Net income (loss)	(39,075)
Ending balance (in shares) at Sep. 30, 2020				27,856,298
Ending balance at Sep. 30, 2020	8,960			$ 28		743,273			(734,341)			0		0
Beginning balance (in shares) at Dec. 31, 2019				27,497,911
Beginning balance at Dec. 31, 2019	$ 19,950			$ 27		715,166			(695,266)			23		0
Increase (Decrease) in Stockholders' Equity
Accounting Standards Update [Extensible List]	espr:AccountingStandardsUpdate202006Member
Ending balance (in shares) at Dec. 31, 2020				25,916,168	25,916,168
Ending balance at Dec. 31, 2020	$ (96,134)	$ (91,927)	$ (188,061)	$ 26	$ 26	797,655	$ (93,475)	$ 704,180	(838,817)	$ 1,548	$ (837,269)	0	$ 0	(54,998)	$ (54,998)
Beginning balance (in shares) at Mar. 31, 2020				27,548,133
Beginning balance at Mar. 31, 2020	(50,246)			$ 28		723,232			(773,515)			9		0
Increase (Decrease) in Stockholders' Equity
Exercise of stock options (in shares)				160,024
Exercise of stock options	3,738					3,738
Vesting of restricted stock units (in shares)				43,498
Vesting of restricted stock units	0
Stock-based compensation	7,395					7,395
Other comprehensive loss	(9)											(9)
Net income (loss)	124,611								124,611
Ending balance (in shares) at Jun. 30, 2020				27,751,655
Ending balance at Jun. 30, 2020	85,489			$ 28		734,365			(648,904)			0		0
Increase (Decrease) in Stockholders' Equity
Exercise of stock options (in shares)				70,578
Exercise of stock options	1,644					1,644
Vesting of restricted stock units (in shares)				34,065
Vesting of restricted stock units	0
Stock-based compensation	7,264					7,264
Net income (loss)	(85,437)								(85,437)
Ending balance (in shares) at Sep. 30, 2020				27,856,298
Ending balance at Sep. 30, 2020	8,960			$ 28		743,273			(734,341)			0		0
Beginning balance (in shares) at Dec. 31, 2020				25,916,168	25,916,168
Beginning balance at Dec. 31, 2020	(96,134)	(91,927)	(188,061)	$ 26	$ 26	797,655	(93,475)	704,180	(838,817)	1,548	(837,269)	0	0	(54,998)	(54,998)
Increase (Decrease) in Stockholders' Equity
Exercise of stock options (in shares)				172,268
Exercise of stock options	2,668					2,668
Vesting of restricted stock units (in shares)				43,465
Vesting of restricted stock units	0
Vesting of ESPP Shares (in shares)				50,818
Vesting of ESPP Shares	1,183					1,183
Stock-based compensation	5,751					5,751
Net income (loss)	(90,935)								(90,935)
Ending balance (in shares) at Mar. 31, 2021				26,182,719
Ending balance at Mar. 31, 2021	(269,394)			$ 26		713,782			(928,204)			0		(54,998)
Beginning balance (in shares) at Dec. 31, 2020				25,916,168	25,916,168
Beginning balance at Dec. 31, 2020	$ (96,134)	$ (91,927)	$ (188,061)	$ 26	$ 26	797,655	$ (93,475)	$ 704,180	(838,817)	$ 1,548	$ (837,269)	0	$ 0	(54,998)	$ (54,998)
Increase (Decrease) in Stockholders' Equity
Exercise of stock options (in shares)	228,741
Net income (loss)	$ (203,989)
Ending balance (in shares) at Sep. 30, 2021				26,800,369
Ending balance at Sep. 30, 2021	(362,705)			$ 27		733,524			(1,041,258)			0		(54,998)
Beginning balance (in shares) at Mar. 31, 2021				26,182,719
Beginning balance at Mar. 31, 2021	(269,394)			$ 26		713,782			(928,204)			0		(54,998)
Increase (Decrease) in Stockholders' Equity
Exercise of stock options (in shares)				10,477
Exercise of stock options	168					168
Vesting of restricted stock units (in shares)				81,139
Vesting of restricted stock units	0
Stock-based compensation	8,584					8,584
Net income (loss)	(43,668)								(43,668)
Ending balance (in shares) at Jun. 30, 2021				26,274,335
Ending balance at Jun. 30, 2021	(304,310)			$ 26		722,534			(971,872)			0		(54,998)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock, net of issuance costs (in shares)				363,061
Issuance of common stock, net of issuance costs	4,207			$ 1		4,206
Exercise of stock options (in shares)				45,996
Exercise of stock options	375					375
Vesting of restricted stock units (in shares)				34,581
Vesting of ESPP Shares (in shares)				82,396
Vesting of ESPP Shares	912					912
Stock-based compensation	5,497					5,497
Net income (loss)	(69,386)								(69,386)
Ending balance (in shares) at Sep. 30, 2021				26,800,369
Ending balance at Sep. 30, 2021	$ (362,705)			$ 27		$ 733,524			$ (1,041,258)			$ 0		$ (54,998)