Cash and Cash Equivalents, and Marketable Securities (ASU 2016-01 and Biocartis) (Details) € in Millions	1 Months Ended		3 Months Ended	6 Months Ended	12 Months Ended
	Dec. 31, 2017 EUR (€) shares	Dec. 31, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jan. 01, 2018 USD ($)	Dec. 31, 2017 USD ($) shares
Marketable securities
Accumulated deficit			$ (227,460,000)	$ (227,460,000)			$ (245,945,000)
Unrealized gain on revaluation of equity investment			1,283,000	1,410,000
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			90,000	218,000
Foreign currency revaluation gain (loss)			$ (231,000)	$ (91,000)
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