Revenue Disaggregated Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 177.8	$ 307.5	$ 524.2	$ 884.3
As reported [Member]
Disaggregation of Revenue [Line Items]
Revenues	175.8	305.6	515.9	875.8
Williston Basin [Member] | As reported [Member]
Disaggregation of Revenue [Line Items]
Revenues	56.6	89.8	164.9	321.4
Other Northern [Member] | As reported [Member]
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.2	1.3	1.4
Permian Basin [Member] | As reported [Member]
Disaggregation of Revenue [Line Items]
Revenues	119.2	215.5	349.7	546.8
Other Southern [Member] | As reported [Member]
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.1	0.0	6.2
Oil and Condensate [Member]
Disaggregation of Revenue [Line Items]
Revenues	169.3	298.8	507.8	834.0
Oil and Condensate [Member] | Williston Basin [Member]
Disaggregation of Revenue [Line Items]
Revenues	58.6	88.9	173.2	306.3
Oil and Condensate [Member] | Other Northern [Member]
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.1	0.9
Oil and Condensate [Member] | Permian Basin [Member]
Disaggregation of Revenue [Line Items]
Revenues	110.7	209.9	334.5	526.7
Oil and Condensate [Member] | Other Southern [Member]
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.1
Natural Gas Liquids [Member]
Disaggregation of Revenue [Line Items]
Revenues	13.2	11.9	29.1	43.1
Natural Gas Liquids [Member] | Williston Basin [Member]
Disaggregation of Revenue [Line Items]
Revenues	4.1	2.4	8.1	15.6
Natural Gas Liquids [Member] | Other Northern [Member]
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.1	0.0	0.1
Natural Gas Liquids [Member] | Permian Basin [Member]
Disaggregation of Revenue [Line Items]
Revenues	9.1	9.4	21.0	27.4
Natural Gas Liquids [Member] | Other Southern [Member]
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Natural Gas [Member]
Disaggregation of Revenue [Line Items]
Revenues	9.3	9.1	24.6	39.4
Natural Gas [Member] | Williston Basin [Member]
Disaggregation of Revenue [Line Items]
Revenues	3.3	5.5	10.9	25.5
Natural Gas [Member] | Other Northern [Member]
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.1	1.2	0.4
Natural Gas [Member] | Permian Basin [Member]
Disaggregation of Revenue [Line Items]
Revenues	6.0	3.4	12.5	7.4
Natural Gas [Member] | Other Southern [Member]
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.1	0.0	6.1
Natural Gas, Gathering, Transportation, Marketing and Processing [Member]
Disaggregation of Revenue [Line Items]
Cost of Goods and Services Sold	(16.0)	(14.2)	(45.6)	(40.7)
Natural Gas, Gathering, Transportation, Marketing and Processing [Member] | Williston Basin [Member]
Disaggregation of Revenue [Line Items]
Cost of Goods and Services Sold	(9.4)	(7.0)	(27.3)	(26.0)
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	0.0
Natural Gas, Gathering, Transportation, Marketing and Processing [Member] | Permian Basin [Member]
Disaggregation of Revenue [Line Items]
Cost of Goods and Services Sold	(6.6)	(7.2)	(18.3)	(14.7)
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	$ 0.0