Operating Revenues - Contract Costs (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Disaggregation of Revenue [Line Items]
Amortization period of contracts costs	10 years
Amortization recorded for deferred costs	$ 3.4
Prepaid expenses and other current assets
Disaggregation of Revenue [Line Items]
Long term deferred costs	11.6
Other assets
Disaggregation of Revenue [Line Items]
Long term deferred costs	$ 21.4