Cash and Cash Equivalents, and Marketable Securities (ASU 2016-01 and Biocartis) (Details) € in Millions	1 Months Ended		3 Months Ended	12 Months Ended
	Dec. 31, 2017 EUR (€) shares	Dec. 31, 2017 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jan. 01, 2018 USD ($)	Dec. 31, 2017 USD ($) shares
Marketable securities
Accumulated deficit			$ (235,777,000)			$ (245,945,000)
Unrealized gain on revaluation of equity investment			127,000
Biocartis N.V.
Marketable securities
Amount invested in shares of common stock	€ 3.4	$ 4,000,000
Estimated fair value, equity securities	€ 3.0					$ 3,500,000
Unrealized losses recorded in other comprehensive income				$ 180,000
Unrealized gain on revaluation of equity investment			127,000
Foreign currency revaluation gain			$ 139,000
Biocartis N.V. | Common Stock
Marketable securities
Number of shares purchased | shares	270,000					270,000
ASU 2016-01
Marketable securities
Accumulated deficit					$ 180,000