Note 6 - Debt (Details Textual) $ / shares in Units, $ in Thousands						3 Months Ended
	Sep. 28, 2018 USD ($) $ / shares shares	Feb. 28, 2018 USD ($) $ / shares shares	Feb. 16, 2018 USD ($)	Sep. 28, 2017 USD ($) $ / shares shares	Mar. 18, 2016 USD ($) $ / shares shares	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)
Proceeds from Issuance of Long-term Debt, Total							$ 11,711
Debt Instrument, Unamortized Discount, Total						3,372		$ 3,619
Debt Issuance Costs Incurred During Period						8
Warrants in Connection with Term Loan A [Member] | Additional Paid-in Capital [Member]
Warrants and Rights Outstanding					$ 200
Warrants in Connection with Term Loan B [Member] | Additional Paid-in Capital [Member]
Warrants and Rights Outstanding				$ 400
Warrants Issued in Connection with Term Loan A and Term Loan B [Member] | Additional Paid-in Capital [Member]
Warrants and Rights Outstanding		$ 500
Warrants in Connection with Term Loan C [Member] | Additional Paid-in Capital [Member]
Warrants and Rights Outstanding	$ 200
Oxford Finance LLC and Silicon Valley Bank [Member] | Warrants in Connection with Term Loan A [Member]
Class of Warrant or Right, Issued During Period | shares					113,066
Class of Warrant or Right, Issued During Period, Exercise Price | $ / shares					$ 5.97
Oxford Finance LLC and Silicon Valley Bank [Member] | Warrants in Connection with Term Loan B [Member]
Class of Warrant or Right, Issued During Period | shares				45,688
Class of Warrant or Right, Issued During Period, Exercise Price | $ / shares				$ 12.31
Oxford Finance LLC and Silicon Valley Bank [Member] | Warrants Issued in Connection with Term Loan A and Term Loan B [Member]
Class of Warrant or Right, Issued During Period | shares		60,490
Class of Warrant or Right, Issued During Period, Exercise Price | $ / shares		$ 9.30
Oxford Finance LLC and Silicon Valley Bank [Member] | Warrants in Connection with Term Loan C [Member]
Class of Warrant or Right, Issued During Period | shares	10,238
Class of Warrant or Right, Issued During Period, Exercise Price | $ / shares	$ 21.98
Oxford Finance LLC and Silicon Valley Bank [Member] | Term Loan A [Member]
Debt Agreement, Maximum Borrowing Capacity					$ 15,000
Oxford Finance LLC and Silicon Valley Bank [Member] | Term Loan B [Member]
Debt Agreement, Maximum Borrowing Capacity				$ 12,500
Debt Issuance Costs Incurred During Period				$ 40
Oxford Finance LLC and Silicon Valley Bank [Member] | Term Loan C [Member]
Debt Agreement, Maximum Borrowing Capacity	$ 5,000
Oxford Finance LLC [Member] | Warrants in Connection with Term Loan A [Member]
Class of Warrant or Right, Issued During Period | shares					56,533
Oxford Finance LLC [Member] | Warrants in Connection with Term Loan B [Member]
Class of Warrant or Right, Issued During Period | shares				22,844
Oxford Finance LLC [Member] | Warrants Issued in Connection with Term Loan A and Term Loan B [Member]
Class of Warrant or Right, Issued During Period | shares		30,245
Silicon Valley Bank [Member] | Warrants in Connection with Term Loan C [Member]
Class of Warrant or Right, Issued During Period | shares	5,119
Term Loans [Member] | Oxford Finance LLC and Silicon Valley Bank [Member]
Debt Agreement, Maximum Borrowing Capacity						$ 45,000
Proceeds from Issuance of Long-term Debt, Total			$ 40,000
Repayments of Long-term Debt, Total			27,500
Loan Amendment Payment			$ 800
Debt Instrument, Unamortized Discount, Total								$ 30
Debt Instrument, Interest Rate Floor						8.65%
Debt Instrument, Interest Rate During Period						9.65%
Debt Instrument, Percentage of Funded Loan Amounts due at Maturity						7.75%
Term Loans [Member] | Oxford Finance LLC and Silicon Valley Bank [Member] | Prime Rate [Member]
Debt Instrument, Basis Spread on Variable Rate						4.15%
Term Loans [Member] | Oxford Finance LLC and Silicon Valley Bank [Member] | Prepayment in First Year After Initial Closing [Member]
Debt Instrument, Prepayment Fee, Amount						$ 1,300
Term Loans [Member] | Oxford Finance LLC and Silicon Valley Bank [Member] | Prepayment in Second Year After Initial Closing [Member]
Debt Instrument, Prepayment Fee Percentage						2.00%
Term Loans [Member] | Oxford Finance LLC and Silicon Valley Bank [Member] | Prepayment After Second Year After Initial Closing [Member]
Debt Instrument, Prepayment Fee Percentage						1.00%
Term Loans [Member] | Oxford Finance LLC and Silicon Valley Bank [Member] | If Term Loan B Funded, Beginning April 2020 [Member]
Debt Instrument, Number of Monthly Principal and Interest Payments						30
Term Loan A [Member] | Oxford Finance LLC and Silicon Valley Bank [Member]
Debt Agreement, Maximum Borrowing Capacity						$ 27,500
Debt Issuance Costs Incurred During Period					$ 1,500
Term Loan B [Member] | Oxford Finance LLC and Silicon Valley Bank [Member]
Debt Agreement, Maximum Borrowing Capacity						12,500
Term Loan C [Member] | Oxford Finance LLC and Silicon Valley Bank [Member]
Debt Agreement, Maximum Borrowing Capacity						$ 5,000