Note 1 - Organization, Description of Business and Summary of Significant Accounting Policies (Details Textual)					3 Months Ended		9 Months Ended		12 Months Ended
	Aug. 07, 2020 USD ($) $ / shares shares	May 13, 2020 USD ($)	May 07, 2019 USD ($) $ / shares shares	Dec. 28, 2018 USD ($)	Sep. 30, 2020 USD ($) $ / shares	Sep. 30, 2019 USD ($) $ / shares	Sep. 30, 2020 USD ($) $ / shares	Sep. 30, 2019 USD ($) $ / shares	Dec. 31, 2019 USD ($)	Mar. 30, 2020 USD ($)
Number of Operating Segments							1
Number of Disease Areas with Genomic Testing					4		4
Stock Issued During Period, Shares, New Issues (in shares) | shares	6,900,000		6,325,000
Sale of Stock, Price Per Share (in dollars per share) | $ / shares	$ 30.00		$ 23.25
Proceeds from Issuance of Common Stock	$ 193,800,000		$ 137,800,000				$ 193,831,000	$ 137,848,000
Payments of Stock Issuance Costs	$ 13,200,000		$ 9,200,000
Equity Securities without Readily Determinable Fair Value, Impairment Loss, Cumulative Amount					$ 0		0
Restricted Cash and Cash Equivalents, Noncurrent, Total					603,000		603,000		$ 603,000
Revenue from Contract with Customer, Excluding Assessed Tax, Total					31,121,000	$ 30,973,000	82,947,000	90,638,000
Operating Income (Loss), Total					$ (4,066,000)	$ (1,763,000)	$ (27,317,000)	$ (6,930,000)
Earnings Per Share, Basic and Diluted, Total (in dollars per share) | $ / shares					$ (0.08)	$ (0.02)	$ (0.52)	$ (0.11)
Inventory Write-down							$ 1,088,000	$ 0
Loxo Oncology Inc [Member] | Collaborative Arrangement [Member]
Contract Agreement, Term (Year)							1 year
Contract Agreement, Automatic Renewal Term (Year)							1 year
Contract Agreement, Termination Period (Day)							90 days
Contract with Customer, Liability, Revenue Recognized							$ 0		0
Johnson and Johnson Services Inc JJSI [Member] | Collaborative Arrangement [Member]
Revenue from Contract with Customer, Excluding Assessed Tax, Total					$ 300,000	$ 2,000,000.0	1,500,000	7,000,000.0
Contract with Customer, Liability, Revenue Recognized							0		0
Collaborative Agreement, Amount				$ 7,000,000.0
Collaborative Agreement, Contingent Receivable Amount				$ 13,000,000.0
Collaborative Agreement, Period of Payments After First Commercial (Year)				10 years
Collaborative Agreement, Fee Percentage of Cash Collections				1.00%
Accounts Receivable, after Allowance for Credit Loss, Total					800,000		800,000		2,000,000.0
Acerta Pharma BV [Member] | Collaborative Arrangement [Member]
Revenue from Contract with Customer, Excluding Assessed Tax, Total					300,000		700,000
Accounts Receivable, after Allowance for Credit Loss, Total					500,000		500,000
Revenue from Achievement of Development Milestones					0		1,000,000.0
CareDx [Member] | Collaborative Arrangement [Member]
Revenue from Contract with Customer, Excluding Assessed Tax, Total					0		1,000,000.0
Accounts Receivable, after Allowance for Credit Loss, Total					0		0
Contract with Customer, Liability, Total		$ 1,000,000.0			0		0
Commercial Development Agreement, Maximum Payments Receivable from Counter Party Upon Achievement of Milestones		4,500,000
Value of Support Services to Counter Party		$ 500,000
Testing [Member]
Revenue from Contract with Customer, Excluding Assessed Tax, Total					28,270,000	26,723,000	70,473,000	78,798,000
Testing [Member] | Cost of Sales [Member] | COVID 19 Pandemic [Member]
Inventory Write-down							1,100,000
Testing [Member] | Revenue Recognition [Member]
Revenue from Contract with Customer, Excluding Assessed Tax, Total					300,000	100,000	1,200,000	800,000
Operating Income (Loss), Total					$ 300,000	$ 100,000	$ 1,200,000	$ 800,000
Earnings Per Share, Basic and Diluted, Total (in dollars per share) | $ / shares					$ 0	$ 0	$ 0.02	$ 0.02
Collaborative [Member]
Revenue from Contract with Customer, Excluding Assessed Tax, Total					$ 0	$ 2,000,000	$ 0	$ 4,000,000
Product [Member]
Revenue from Contract with Customer, Excluding Assessed Tax, Total					2,027,000	0	7,149,000	0
Product [Member] | Loxo Oncology Inc [Member] | Collaborative Arrangement [Member]
Contract with Customer, Liability, Revenue Recognized							0	90,000
Service [Member] | Loxo Oncology Inc [Member] | Collaborative Arrangement [Member]
Contract with Customer, Liability, Revenue Recognized					300,000		800,000
Fulfillment of Obligations Relating to Percepta v.2 Development Milestones [Member] | Johnson and Johnson Services Inc JJSI [Member] | Collaborative Arrangement [Member]
Revenue from Contract with Customer, Excluding Assessed Tax, Total						$ 2,000,000.0		4,000,000.0
Other Noncurrent Assets [Member]
Equity Securities without Readily Determinable Fair Value, Amount					$ 1,000,000.0		$ 1,000,000.0
Overnight Reverse Repurchase Agreements [Member]
Cash Equivalents, Percentage Collateralized by US Treasury and Agency Securities					102.00%		102.00%
Cash Equivalents, at Carrying Value, Total					$ 0		$ 0
Collateral for Irrevocable Standby Letter of Credit [Member]
Restricted Cash and Cash Equivalents, Noncurrent, Total									$ 603,000	$ 603,000
Over-Allotment Option [Member]
Stock Issued During Period, Shares, New Issues (in shares) | shares	900,000		825,000
Revision of Prior Period, Adjustment [Member]
Increase (Decrease) in Operating Capital, Total								$ 810,000