Property and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property and Equipment
Property and equipment, gross	$ 62,871	$ 56,872
Less: accumulated depreciation	(49,695)	(46,715)
Property and equipment, net	13,176	10,157
Depreciation expense	3,000	2,500	$ 2,200
Computer equipment
Property and Equipment
Property and equipment, gross	2,522	1,866
Software
Property and Equipment
Property and equipment, gross	3,577	3,432
Furniture and fixtures
Property and Equipment
Property and equipment, gross	3,759	3,759
Laboratory equipment
Property and Equipment
Property and equipment, gross	31,164	28,371
Leasehold improvements
Property and Equipment
Property and equipment, gross	$ 21,849	$ 19,444

Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.

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The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation.

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Amount before accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

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Amount after accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business to produce goods and services and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

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