Operating Revenues - Contract Costs (Details) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)
Disaggregation of Revenue [Line Items]
Amortization period of contracts costs		10 years
Capitalized Contract Cost, Amortization	$ 3.8	$ 10.7
Prepaid expenses and other current assets
Disaggregation of Revenue [Line Items]
Long term deferred costs	10.5	10.5
Other assets
Disaggregation of Revenue [Line Items]
Long term deferred costs	$ 21.0	$ 21.0