CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Common stock	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Treasury Stock [Member]	Total Amphastar Stockholders' Equity [Member]	Non-controlling Interest [Member]	Total
Balance at Dec. 31, 2018	$ 5	$ 344,434	$ 67,485	$ (4,013)	$ (75,476)	$ 332,435	$ 31,924
Balance at Dec. 31, 2018								$ 364,359
Balance at Dec. 31, 2018	51,438,675				(4,807,557)
Changes in Stockholders' Equity
Beginning balance adjustment as a result of the adoption of accounting standards			(54)			(54)		(54)
Net income (loss) attributable to Amphastar Pharmaceuticals, Inc.			868			868		868
Other comprehensive income (loss) attributable to Amphastar Pharmaceuticals, Inc.				(113)		(113)		(113)
Proceeds from the private placement of ANP		2,588				2,588	16,378	18,966
Net loss attributable to non-controlling interest							(3,022)	(3,022)
Treasury stock acquired					$ (3,015)	(3,015)		(3,015)
Treasury stock acquired (in Shares)					(145,479)
Issuance of treasury stock in connection with the Company's equity plans		(98)			$ 98
Issuance of treasury stock in connection with the Company's equity plans (in Shares)					8,334
Issuance of common stock in connection with the Company's equity plans		(2,397)				(2,397)		(2,397)
Issuance of common stock in connection with the Company's equity plans (in Shares)	604,651
Share-based compensation expense		4,674				4,674		4,674
Balance at Mar. 31, 2019	$ 5	349,201	68,299	(4,126)	$ (78,393)	334,986	45,280
Balance at Mar. 31, 2019								380,266
Balance at Mar. 31, 2019	52,043,326				(4,944,702)
Balance at Dec. 31, 2018	$ 5	344,434	67,485	(4,013)	$ (75,476)	332,435	31,924
Balance at Dec. 31, 2018								364,359
Balance at Dec. 31, 2018	51,438,675				(4,807,557)
Changes in Stockholders' Equity
Net income (loss) attributable to Amphastar Pharmaceuticals, Inc.								48,655
Other comprehensive income (loss) attributable to Amphastar Pharmaceuticals, Inc.								(210)
Balance at Jun. 30, 2019	$ 5	355,436	116,086	(4,223)	$ (79,459)	387,845	44,443
Balance at Jun. 30, 2019								432,288
Balance at Jun. 30, 2019	52,212,760				(4,995,085)
Balance at Mar. 31, 2019	$ 5	349,201	68,299	(4,126)	$ (78,393)	334,986	45,280
Balance at Mar. 31, 2019								380,266
Balance at Mar. 31, 2019	52,043,326				(4,944,702)
Changes in Stockholders' Equity
Net income (loss) attributable to Amphastar Pharmaceuticals, Inc.			47,787			47,787		47,787
Other comprehensive income (loss) attributable to Amphastar Pharmaceuticals, Inc.				(97)		(97)		(97)
Net loss attributable to non-controlling interest							(867)	(867)
Treasury stock acquired					$ (1,073)	(1,073)		(1,073)
Treasury stock acquired (in Shares)					(50,980)
Issuance of treasury stock in connection with the Company's equity plans		(7)			$ 7
Issuance of treasury stock in connection with the Company's equity plans (in Shares)					597
Issuance of common stock in connection with the Company's equity plans		2,240				2,240		2,240
Issuance of common stock in connection with the Company's equity plans (in Shares)	169,434
Share-based compensation expense		4,002				4,002	30	4,032
Balance at Jun. 30, 2019	$ 5	355,436	116,086	(4,223)	$ (79,459)	387,845	44,443
Balance at Jun. 30, 2019								432,288
Balance at Jun. 30, 2019	52,212,760				(4,995,085)
Balance at Dec. 31, 2019	$ 5	367,305	116,370	(4,687)	$ (97,627)	381,366	46,162	381,366
Balance at Dec. 31, 2019								$ 427,528
Balance at Dec. 31, 2019	52,495,483				(5,918,515)			46,576,968
Changes in Stockholders' Equity
Net income (loss) attributable to Amphastar Pharmaceuticals, Inc.			3,949			3,949		$ 3,949
Other comprehensive income (loss) attributable to Amphastar Pharmaceuticals, Inc.				(774)		(774)		(774)
Net loss attributable to non-controlling interest							(424)	(424)
Treasury stock acquired					$ (10,950)	(10,950)		(10,950)
Treasury stock acquired (in Shares)					(647,246)
Issuance of treasury stock in connection with the Company's equity plans		(84)			$ 84
Issuance of treasury stock in connection with the Company's equity plans (in Shares)					6,873
Issuance of common stock in connection with the Company's equity plans		(1,238)				(1,238)		(1,238)
Issuance of common stock in connection with the Company's equity plans (in Shares)	369,508
Share-based compensation expense		5,161				5,161	121	5,282
Balance at Mar. 31, 2020	$ 5	371,144	120,319	(5,461)	$ (108,493)	377,514	45,859
Balance at Mar. 31, 2020								423,373
Balance at Mar. 31, 2020	52,864,991				(6,558,888)
Balance at Dec. 31, 2019	$ 5	367,305	116,370	(4,687)	$ (97,627)	381,366	46,162	381,366
Balance at Dec. 31, 2019								$ 427,528
Balance at Dec. 31, 2019	52,495,483				(5,918,515)			46,576,968
Changes in Stockholders' Equity
Net income (loss) attributable to Amphastar Pharmaceuticals, Inc.								$ 3,757
Other comprehensive income (loss) attributable to Amphastar Pharmaceuticals, Inc.								(486)
Balance at Jun. 30, 2020	$ 5	396,841	120,127	(5,173)	$ (114,119)	397,681	44,756	397,681
Balance at Jun. 30, 2020								$ 442,437
Balance at Jun. 30, 2020	54,372,275				(6,877,366)			47,494,909
Balance at Mar. 31, 2020	$ 5	371,144	120,319	(5,461)	$ (108,493)	377,514	45,859
Balance at Mar. 31, 2020								$ 423,373
Balance at Mar. 31, 2020	52,864,991				(6,558,888)
Changes in Stockholders' Equity
Net income (loss) attributable to Amphastar Pharmaceuticals, Inc.			(192)			(192)		(192)
Other comprehensive income (loss) attributable to Amphastar Pharmaceuticals, Inc.				288		288		288
Net loss attributable to non-controlling interest							(1,237)	(1,237)
Treasury stock acquired					$ (5,756)	(5,756)		(5,756)
Treasury stock acquired (in Shares)					(329,391)
Issuance of treasury stock in connection with the Company's equity plans		(130)			$ 130
Issuance of treasury stock in connection with the Company's equity plans (in Shares)					10,913
Issuance of common stock in connection with the Company's equity plans		19,448				19,448		19,448
Issuance of common stock in connection with the Company's equity plans (in Shares)	1,507,284
Share-based compensation expense		6,379				6,379	134	6,513
Balance at Jun. 30, 2020	$ 5	$ 396,841	$ 120,127	$ (5,173)	$ (114,119)	$ 397,681	$ 44,756	397,681
Balance at Jun. 30, 2020								$ 442,437
Balance at Jun. 30, 2020	54,372,275				(6,877,366)			47,494,909