Operating Revenues - Contract Costs (Details) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)
Disaggregation of Revenue [Line Items]
Amortization period of contracts costs		10 years
Capitalized Contract Cost, Amortization	$ 3.5	$ 6.9
Prepaid expenses and other current assets
Disaggregation of Revenue [Line Items]
Long term deferred costs	11.6	11.6
Other assets
Disaggregation of Revenue [Line Items]
Long term deferred costs	$ 21.1	$ 21.1