Income tax Income tax (Details) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Deferred Tax Assets, Operating Loss Carryforwards, Domestic	$ 57,000,000	$ 57,000,000	$ 50,000,000
SEC Schedule, 12-09, Valuation Allowances and Reserves, Amount	0	0
Unrecognized Tax Benefits, Income Tax Penalties Accrued	0	0	$ 0
Customer Refund Liability, Noncurrent	6,000,000	6,000,000
Revenue Subject to Refund [Member]
Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount	$ 11,000,000	$ 36,000,000