Revenues Performance Obligations (Details) $ in Millions	6 Months Ended
	Jun. 30, 2019 USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Fixed Fee Revenues Recognized	$ 607.1
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-07-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	6 months
Estimated revenues from contracts with customers from unsatisfied performance obligations as of June 30, 2019 (1)	$ 1,123.0	[1]
Operating revenues which are fixed and determinable (operating leases) (1)	20.0	[1]
Total projected operating revenues under committed firm agreements as of June 30, 2019 (1)	$ 1,143.0	[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 June 30, 2019 (1)	$ 989.5	[1]
Operating revenues which are fixed and determinable (operating leases) (1)	20.5	[1]
Total projected operating revenues under committed firm agreements as of June 30, 2019 (1)	1,010.0	[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 June 30, 2019 (1)	7,879.5	[1]
Operating revenues which are fixed and determinable (operating leases) (1)	187.5	[1]
Total projected operating revenues under committed firm agreements as of June 30, 2019 (1)	8,067.0	[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 June 30, 2019 (1)	9,992.0	[1]
Operating revenues which are fixed and determinable (operating leases) (1)	228.0	[1]
Total projected operating revenues under committed firm agreements as of June 30, 2019 (1)	$ 10,220.0	[1]

[1] For the 2019 period, $607.1 million represents actual fixed fee revenues recognized for the fulfillment of performance obligations during the six months ended June 30, 2019.