CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Common Stock [Member]	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.			48,939			48,939		48,939
Other comprehensive income (loss) attributable to Amphastar Pharmaceuticals, Inc.				(674)		(674)		(674)
Proceeds from the private placement of ANP		2,588				2,588	16,378	18,966
Net income (loss) attributable to non-controlling interest							(2,434)	(2,434)
Purchase of treasury stock					$ (22,291)	(22,291)		(22,291)
Purchase of treasury stock (in shares)					(1,122,781)
Issuance of treasury stock in connection with the Company's equity plans		(140)			$ 140
Issuance of treasury stock in connection with the Company's equity plans (in Shares)					11,823
Issuance of common stock in connection with the Company's equity plans		3,421				3,421		3,421
Issuance of common stock in connection with the Company's equity plans (in Shares)	1,056,808
Share-based compensation expense		17,001				17,001	294	17,295
Balance at Dec. 31, 2019	$ 5	367,305	116,370	(4,687)	$ (97,627)	381,366	46,162
Balance at Dec. 31, 2019								427,528
Balance at Dec. 31, 2019	52,495,483				(5,918,515)
Changes in Stockholders' Equity
Net income (loss) attributable to Amphastar Pharmaceuticals, Inc.			1,403			1,403		1,403
Other comprehensive income (loss) attributable to Amphastar Pharmaceuticals, Inc.				966		966		966
ANP Restructuring							(106)	(106)
Net income (loss) attributable to non-controlling interest							(306)	(306)
Purchase of treasury stock					$ (24,425)	(24,425)		(24,425)
Purchase of treasury stock (in shares)					(1,366,915)
Issuance of treasury stock in connection with the Company's equity plans		(240)			$ 240
Issuance of treasury stock in connection with the Company's equity plans (in Shares)					19,947
Issuance of common stock in connection with the Company's equity plans		23,165				23,165		23,165
Issuance of common stock in connection with the Company's equity plans (in Shares)	2,265,439
Share-based compensation expense		19,831				19,831	667	20,498
Balance at Dec. 31, 2020	$ 5	410,061	117,773	(3,721)	$ (121,812)	402,306	46,417	402,306
Balance at Dec. 31, 2020								$ 448,723
Balance at Dec. 31, 2020	54,760,922				(7,265,483)			47,495,439
Changes in Stockholders' Equity
Net income (loss) attributable to Amphastar Pharmaceuticals, Inc.			62,116			62,116		$ 62,116
Other comprehensive income (loss) attributable to Amphastar Pharmaceuticals, Inc.								(3,044)
Other comprehensive income (loss) attributable to Amphastar Pharmaceuticals, Inc.				(2,682)		(2,682)		(2,682)
ANP Restructuring		(22,162)	448	(362)		(22,076)	(46,641)	(68,717)
Net income (loss) attributable to non-controlling interest							1,185	1,185
Purchase of treasury stock					$ (28,873)	(28,873)		(28,873)
Purchase of treasury stock (in shares)					(1,477,305)
Issuance of treasury stock in connection with the Company's equity plans		(206)			$ 206
Issuance of treasury stock in connection with the Company's equity plans (in Shares)					17,498
Issuance of common stock in connection with the Company's equity plans	$ 1	15,924				15,925		15,925
Issuance of common stock in connection with the Company's equity plans (in Shares)	1,679,280
Share-based compensation expense		18,806				18,806	$ (961)	17,845
Balance at Dec. 31, 2021	$ 6	$ 422,423	$ 180,337	$ (6,765)	$ (150,479)	$ 445,522		445,522
Balance at Dec. 31, 2021								$ 445,522
Balance at Dec. 31, 2021	56,440,202				(8,725,290)			47,714,912