Revenue Costs to obtain or fulfill a contract (Details) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)
Revenue Recognition [Abstract]
Costs to fulfill a contract	$ 14.3	$ 14.3
Amortization of costs to fulfill a contract	$ 1.2	$ 2.5