CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Sanofi Genzyme	Common Stock	Common Stock Sanofi Genzyme	Additional Paid-in Capital	Additional Paid-in Capital Sanofi Genzyme	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Balance at Dec. 31, 2014	$ 936,267		$ 772		$ 1,843,362		$ 48,763	$ (956,630)
Balance (in shares) at Dec. 31, 2014			77,202,753
Exercise of common stock options, net of tax withholdings	29,425		$ 15		29,410
Exercise of common stock options, net of tax withholdings (in shares)			1,461,237
Issuance of common stock under other types of equity plans	2,666		$ 1		2,665
Issuance of common stock under other types of equity plans (in shares)			32,427
Issuance of common stock under equity plans, net of tax withholdings	(378)				(378)
Issuance of common stock under equity plans, net of tax withholdings (in shares)			6,366
Issuance of common stock, net of offering costs	496,400	$ 89,018	$ 54	$ 9	496,346	$ 89,009
Issuance of common stock, net of offering costs (in shares)			5,447,368	940,817
Stock-based compensation expense related to equity-classified awards	45,783				45,783
Other comprehensive loss, net of tax	(44,394)						(44,394)
Net loss	(290,073)							(290,073)
Balance at Dec. 31, 2015	1,264,714		$ 851		2,506,197		4,369	(1,246,703)
Balance (in shares) at Dec. 31, 2015			85,090,968
Exercise of common stock options, net of tax withholdings	11,608		$ 5		11,603
Exercise of common stock options, net of tax withholdings (in shares)			559,344
Issuance of common stock under other types of equity plans	3,648		$ 1		3,647
Issuance of common stock under other types of equity plans (in shares)			75,453
Issuance of common stock under equity plans, net of tax withholdings	(314)				(314)
Issuance of common stock under equity plans, net of tax withholdings (in shares)			10,549
Issuance of common stock, net of offering costs		14,301		$ 2		14,299
Issuance of common stock, net of offering costs (in shares)				205,030
Stock-based compensation expense related to equity-classified awards	74,182				74,182
Other comprehensive loss, net of tax	(37,810)						(37,810)
Reclassification adjustment for realized (gain) loss on marketable securities included in net loss	(6,977)
Net loss	(410,108)							(410,108)
Balance at Dec. 31, 2016	$ 920,221		$ 859		2,609,614		(33,441)	(1,656,811)
Balance (in shares) at Dec. 31, 2016	85,941,344		85,941,344
Exercise of common stock options, net of tax withholdings	$ 80,546		$ 19		80,527
Exercise of common stock options, net of tax withholdings (in shares)			1,841,566
Issuance of common stock under other types of equity plans	5,907		$ 2		5,905
Issuance of common stock under other types of equity plans (in shares)			134,615
Issuance of common stock under equity plans, net of tax withholdings	(707)				(707)
Issuance of common stock under equity plans, net of tax withholdings (in shares)			11,523
Issuance of common stock, net of offering costs	1,139,625	$ 21,381	$ 114	$ 3	1,139,511	$ 21,378
Issuance of common stock, net of offering costs (in shares)			11,440,000	297,501
Stock-based compensation expense related to equity-classified awards	91,324				91,324
Other comprehensive loss, net of tax	(992)						(992)
Reclassification adjustment for realized (gain) loss on marketable securities included in net loss	1,894
Net loss	(490,874)							(490,874)
Balance at Dec. 31, 2017	$ 1,766,431		$ 997		$ 3,947,552		$ (34,433)	$ (2,147,685)
Balance (in shares) at Dec. 31, 2017	99,666,549		99,666,549