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Acquisitions - Additional Information (Detail) - Counsyl Inc [Member]
$ / shares in Units, $ in Millions
Jul. 31, 2018
USD ($)
$ / shares
shares
Business Acquisition [Line Items]  
Business acquisition consideration $ 405.9
Acquisition costs 6.8
Acquired intangible assets 290.0
Developed Screening Processes [Member]  
Business Acquisition [Line Items]  
Acquired intangible assets $ 290.0
Intangible assets estimated useful lives 12 years
Developed Screening Processes [Member] | Measurement Input, Discount Rate [Member]  
Business Acquisition [Line Items]  
Estimated net cash flows, discount rate 12.5
Agreement and Plan of Merger [Member]  
Business Acquisition [Line Items]  
Effective date of business acquisition Jul. 31, 2018
Business acquisition consideration $ 405.9
Cash payment on business acquisition 278.5
Amount of cash consideration deposited in escrow account $ 5.0
Agreement and Plan of Merger [Member] | Common Stock [Member]  
Business Acquisition [Line Items]  
Business acquisition, shares issued | shares 2,994,251
Business acquisition, shares issued, value $ 127.4
Business acquisition, per share market closing price | $ / shares $ 42.53