Related Party Transactions - Additional Information (Detail) - USD ($)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Accounts receivable	$ 10,159,000	$ 2,007,000
Roche related-party deferred revenue	3,742,000	3,747,000
Roche [Member]
Related Party Transaction [Line Items]
Amount of related party transaction during the period	0	0	$ 0
Roche [Member] \| Molecular Information Platform Program and x-U.S. Commercialization Agreement [Member] \| Molecular Information Services [Member]
Related Party Transaction [Line Items]
Recognized revenue	25,458,000	21,493,000	13,132,000
Cost of related-party molecular information services from Roche	9,014,000	3,770,000	2,736,000
Roche [Member] \| CDx Development, Immunotherapy Testing Platform Development and Other Programs [Member] \| Pharma Research and Development Services [Member]
Related Party Transaction [Line Items]
Recognized revenue	24,828,000	21,984,000	312,000
Former Member of Board of Directors [Member]
Related Party Transaction [Line Items]
Recognized revenue	481,000	1,631,000	$ 1,716,000
Accounts receivable	$ 0	$ 0

Deferred revenue related party.

No definition available.

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- Definition

Amount of receivables arising from transactions with related parties due within one year or the normal operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
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- Definition

Costs of sales and operating expenses for the period incurred from transactions with related parties.

+ References

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

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Amount of revenue, fees and commissions earned from transactions between (a) a parent company and its subsidiaries; (b) subsidiaries of a common parent; (c) an entity and trusts for the benefit of employees, for example, but not limited to, pension and profit-sharing trusts that are managed by or under the trusteeship of the entity's management; (d) an entity and its principal, owners, management, or members of their immediate families; and (e) affiliates.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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