Acquisitions - Schedule of Unaudited Pro forma Information (Detail) - Genoptix - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Revenue	$ 367,988	$ 356,711
Net income (loss) attributable to common stockholders	$ 1,401	$ (42,930)
Income (loss) per share (in dollars per share)	$ 0.02	$ (0.53)
Basic (in shares)	85,618	80,426
Diluted (in shares)	91,568	80,426