Revenue (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 200.2	$ 177.8	$ 120.6	$ 225.8	$ 321.9	$ 307.5	$ 296.2	$ 280.6	$ 724.4	$ 1,206.2
As reported [Member]
Disaggregation of Revenue [Line Items]
Revenues									714.6	1,187.4	$ 1,871.3
Williston Basin [Member] | As reported [Member]
Disaggregation of Revenue [Line Items]
Revenues									244.6	438.9	765.7
Other Northern [Member] | As reported [Member]
Disaggregation of Revenue [Line Items]
Revenues									1.3	1.6	6.9
Permian Basin [Member] | As reported [Member]
Disaggregation of Revenue [Line Items]
Revenues									468.7	740.7	739.3
Haynesville [Member]
Disaggregation of Revenue [Line Items]
Revenues										5.9
Haynesville [Member] | As reported [Member]
Disaggregation of Revenue [Line Items]
Revenues											304.1
Other Southern [Member] | As reported [Member]
Disaggregation of Revenue [Line Items]
Revenues									0.0	6.2	0.2
Uinta Basin [Member] | As reported [Member]
Disaggregation of Revenue [Line Items]
Revenues											55.1
Oil and Condensate [Member]
Disaggregation of Revenue [Line Items]
Revenues									691.8	1,132.6	1,422.4
Oil and Condensate [Member] | Williston Basin [Member]
Disaggregation of Revenue [Line Items]
Revenues									249.9	420.8	707.0
Oil and Condensate [Member] | Other Northern [Member]
Disaggregation of Revenue [Line Items]
Revenues									0.2	1.1	4.9
Oil and Condensate [Member] | Permian Basin [Member]
Disaggregation of Revenue [Line Items]
Revenues									441.7	710.6	684.4
Oil and Condensate [Member] | Haynesville [Member]
Disaggregation of Revenue [Line Items]
Revenues											1.0
Oil and Condensate [Member] | Other Southern [Member]
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.1	(0.2)
Oil and Condensate [Member] | Uinta Basin [Member]
Disaggregation of Revenue [Line Items]
Revenues											25.3
Natural Gas [Member]
Disaggregation of Revenue [Line Items]
Revenues									39.6	52.4	393.1
Natural Gas [Member] | Williston Basin [Member]
Disaggregation of Revenue [Line Items]
Revenues									18.1	33.1	45.3
Natural Gas [Member] | Other Northern [Member]
Disaggregation of Revenue [Line Items]
Revenues									1.1	0.4	2.0
Natural Gas [Member] | Permian Basin [Member]
Disaggregation of Revenue [Line Items]
Revenues									20.4	12.8	17.3
Natural Gas [Member] | Haynesville [Member]
Disaggregation of Revenue [Line Items]
Revenues											303.1
Natural Gas [Member] | Other Southern [Member]
Disaggregation of Revenue [Line Items]
Revenues									0.0	6.1	0.4
Natural Gas [Member] | Uinta Basin [Member]
Disaggregation of Revenue [Line Items]
Revenues											25.0
Natural Gas Liquids [Member]
Disaggregation of Revenue [Line Items]
Revenues									45.7	57.3	110.8
Natural Gas Liquids [Member] | Williston Basin [Member]
Disaggregation of Revenue [Line Items]
Revenues									14.4	19.4	56.5
Natural Gas Liquids [Member] | Other Northern [Member]
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.1	0.0
Natural Gas Liquids [Member] | Permian Basin [Member]
Disaggregation of Revenue [Line Items]
Revenues									31.3	37.8	49.5
Natural Gas Liquids [Member] | Haynesville [Member]
Disaggregation of Revenue [Line Items]
Revenues											0.0
Natural Gas Liquids [Member] | Other Southern [Member]
Disaggregation of Revenue [Line Items]
Revenues									0.0	0.0	0.0
Natural Gas Liquids [Member] | Uinta Basin [Member]
Disaggregation of Revenue [Line Items]
Revenues											4.8
Natural Gas, Gathering, Transportation, Marketing and Processing [Member]
Disaggregation of Revenue [Line Items]
Cost of Goods and Services Sold									(62.5)	(54.9)	(55.0)
Natural Gas, Gathering, Transportation, Marketing and Processing [Member] | Williston Basin [Member]
Disaggregation of Revenue [Line Items]
Cost of Goods and Services Sold									(37.8)	(34.4)	(43.1)
Natural Gas, Gathering, Transportation, Marketing and Processing [Member] | Other Northern [Member]
Disaggregation of Revenue [Line Items]
Cost of Goods and Services Sold									0.0	0.0	0.0
Natural Gas, Gathering, Transportation, Marketing and Processing [Member] | Permian Basin [Member]
Disaggregation of Revenue [Line Items]
Cost of Goods and Services Sold									(24.7)	(20.5)	(11.9)
Natural Gas, Gathering, Transportation, Marketing and Processing [Member] | Haynesville [Member]
Disaggregation of Revenue [Line Items]
Cost of Goods and Services Sold											0.0
Natural Gas, Gathering, Transportation, Marketing and Processing [Member] | Other Southern [Member]
Disaggregation of Revenue [Line Items]
Cost of Goods and Services Sold									$ 0.0	$ 0.0	0.0
Natural Gas, Gathering, Transportation, Marketing and Processing [Member] | Uinta Basin [Member]
Disaggregation of Revenue [Line Items]
Cost of Goods and Services Sold											$ 0.0