Revenues Performance Obligations (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Fixed Fee Revenues Recognized	$ 308.7
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-04-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	9 months
Estimated revenues from contracts with customers from unsatisfied performance obligations as of March 31, 2019	$ 1,095.0
Operating revenues which are fixed and determinable (operating leases)	19.0
Total projected operating revenues under committed firm agreements	$ 1,114.0	[1]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year
Estimated revenues from contracts with customers from unsatisfied performance obligations as of March 31, 2019	$ 970.0
Operating revenues which are fixed and determinable (operating leases)	21.0
Total projected operating revenues under committed firm agreements	991.0	[1]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Estimated revenues from contracts with customers from unsatisfied performance obligations as of March 31, 2019	7,495.0
Operating revenues which are fixed and determinable (operating leases)	208.0
Total projected operating revenues under committed firm agreements	7,703.0	[1]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: (nil)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Estimated revenues from contracts with customers from unsatisfied performance obligations as of March 31, 2019	9,560.0
Operating revenues which are fixed and determinable (operating leases)	248.0
Total projected operating revenues under committed firm agreements	$ 9,808.0	[1]

[1] For the 2019 period, $308.7 million represents actual fixed fee revenues recognized for the fulfillment of performance obligations during the three months ended March 31, 2019.