Fair Value Measurements - Narrative (Detail) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended
	Feb. 28, 2018	Aug. 31, 2017	Sep. 30, 2018	Mar. 31, 2018	Jun. 30, 2018	May 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities, current			$ 25,114				$ 4,452
Amended Credit Facility
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Credit facility repayment feature			5,600		$ 2,600	$ 500
Cash and cash equivalent threshold			$ 50,000			$ 50,000
Good Start Genetics, Inc.
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of shares received (in shares)		414,237	78,103
Invitae Corporation
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of shares received (in shares)		414,237
Proceeds from sale of trading securities held-for-investment	$ 2,600			$ 2,600