Revenue Disaggregated Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 120.6	$ 296.2	$ 346.4	$ 576.8
As reported [Member]
Disaggregation of Revenue [Line Items]
Revenues	118.3	294.6	340.1	570.2
Williston Basin [Member] | As reported [Member]
Disaggregation of Revenue [Line Items]
Revenues	30.3	111.9	108.3	231.6
Other Northern [Member] | As reported [Member]
Disaggregation of Revenue [Line Items]
Revenues	1.1	0.6	1.3	1.2
Permian Basin [Member] | As reported [Member]
Disaggregation of Revenue [Line Items]
Revenues	86.9	181.7	230.5	331.3
Other Southern [Member] | As reported [Member]
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.4	0.0	6.1
Oil and Condensate [Member]
Disaggregation of Revenue [Line Items]
Revenues	118.5	285.7	338.5	535.2
Oil and Condensate [Member] | Williston Basin [Member]
Disaggregation of Revenue [Line Items]
Revenues	35.4	107.5	114.6	217.4
Oil and Condensate [Member] | Other Northern [Member]
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.5	0.1	0.9
Oil and Condensate [Member] | Permian Basin [Member]
Disaggregation of Revenue [Line Items]
Revenues	83.1	177.6	223.8	316.8
Oil and Condensate [Member] | Other Southern [Member]
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.1	0.0	0.1
Natural Gas [Member]
Disaggregation of Revenue [Line Items]
Revenues	8.8	7.3	15.3	30.3
Natural Gas [Member] | Williston Basin [Member]
Disaggregation of Revenue [Line Items]
Revenues	3.0	7.5	7.6	20.0
Natural Gas [Member] | Other Northern [Member]
Disaggregation of Revenue [Line Items]
Revenues	1.1	0.1	1.2	0.3
Natural Gas [Member] | Permian Basin [Member]
Disaggregation of Revenue [Line Items]
Revenues	4.7	(0.6)	6.5	4.0
Natural Gas [Member] | Other Southern [Member]
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.3	0.0	6.0
Natural Gas Liquids [Member]
Disaggregation of Revenue [Line Items]
Revenues	6.3	14.3	15.9	31.2
Natural Gas Liquids [Member] | Williston Basin [Member]
Disaggregation of Revenue [Line Items]
Revenues	0.5	5.8	4.0	13.2
Natural Gas Liquids [Member] | Other Northern [Member]
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Natural Gas Liquids [Member] | Permian Basin [Member]
Disaggregation of Revenue [Line Items]
Revenues	5.8	8.5	11.9	18.0
Natural Gas Liquids [Member] | Other Southern [Member]
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Natural Gas, Gathering, Transportation, Marketing and Processing [Member]
Disaggregation of Revenue [Line Items]
Cost of Goods and Services Sold	(15.3)	(12.7)	(29.6)	(26.5)
Natural Gas, Gathering, Transportation, Marketing and Processing [Member] | Williston Basin [Member]
Disaggregation of Revenue [Line Items]
Cost of Goods and Services Sold	(8.6)	(8.9)	(17.9)	(19.0)
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	(6.7)	(3.8)	(11.7)	(7.5)
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