Income tax Income tax (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Deferred Tax Assets, Operating Loss Carryforwards, Domestic	$ 55,000,000		$ 55,000,000		$ 50,000,000
Valuation Allowances and Reserves, Balance	0		0
Revenues	449,000,000	$ 449,000,000	942,000,000	$ 979,000,000
Unrecognized Tax Benefits, Income Tax Penalties Accrued	0		0		$ 0
Customer Refund Liability, Noncurrent	$ 4,000,000		4,000,000
Revenue Subject to Refund [Member]
Revenues			$ 25,000,000