Condensed Consolidated Statements of Comprehensive Loss (Unaudited) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended		141 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012
Condensed Consolidated Statements of Comprehensive Loss [Abstract]
Revenue	$ 729	$ 1,259	$ 3,341	$ 3,776	$ 68,822
Operating expenses:
Research and development	4,784	8,051	14,826	20,977	321,077
General and administrative	2,338	3,109	6,487	8,664	110,363
Restructuring charges					2,037
Acquired in-process research and development					3,916
Total operating expenses	7,122	11,160	21,313	29,641	437,393
Loss from operations	(6,393)	(9,901)	(17,972)	(25,865)	(368,571)
Loss/ (gain) on change in fair value of contingent consideration liability	(200)	(3,000)	1,000	(3,300)	(6,145)
Interest and other income/ (expense), net	5	13	413	30	14,302
Interest expense	(333)	(529)	(1,146)	(1,703)	(9,015)
Net loss	(6,921)	(13,417)	(17,705)	(30,838)	(369,429)
Consideration paid in excess of noncontrolling interest					(61,566)
Net loss attributed to noncontrolling interest in Symphony Allegro, Inc.					45,089
Net loss attributable to Alexza common stockholders	(6,921)	(13,417)	(17,705)	(30,838)	(385,906)
Basic and diluted net loss per share attributable to Alexza common stockholders	$ (0.52)	$ (1.86)	$ (1.55)	$ (4.64)
Shares used to compute basic and diluted net loss per share attributable to Alexza common stockholders	13,235	7,213	11,394	6,644
Change in unrealized gain (loss) on marketable securities		(15)
Comprehensive loss attributed to Alexza common stockholders	$ (6,921)	$ (13,432)	$ (17,705)	$ (30,838)	$ (385,906)