Other balance sheet items - Schedule of Trade and Other Receivables, Net (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Jun. 30, 2020	Jan. 01, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross trade and other receivables	$ 73,015,000	$ 27,447,000
Allowance for doubtful accounts	(1,765,000)	(580,000)
Total trade and other receivables, net	71,250,000	26,867,000	$ 64,800,000
Additional bad debt expense recognized	800,000
Business Interruption Insurance Claims Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross trade and other receivables	$ 0	$ 3,000,000.0