Revenue Disaggregated Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Revenue from Contract with Customer, Including Assessed Tax	$ 296.2	$ 532.4	$ 576.8	$ 961.3
Oil and Condensate [Member] | As reported [Member] | Williston Basin [Member]
Revenue from Contract with Customer, Including Assessed Tax	107.5	207.6	217.4	368.1
Oil and Condensate [Member] | As reported [Member] | Uinta Basin [Member]
Revenue from Contract with Customer, Including Assessed Tax		9.5		17.9
Oil and Condensate [Member] | As reported [Member] | Other Northern [Member]
Revenue from Contract with Customer, Including Assessed Tax	0.5	0.9	0.9	2.8
Oil and Condensate [Member] | As reported [Member] | Permian Basin [Member]
Revenue from Contract with Customer, Including Assessed Tax	177.6	190.3	316.8	320.1
Oil and Condensate [Member] | As reported [Member] | Haynesville/Cotton Valley [Member]
Revenue from Contract with Customer, Including Assessed Tax		0.2		0.6
Oil and Condensate [Member] | As reported [Member] | Other Southern [Member]
Revenue from Contract with Customer, Including Assessed Tax	0.1	0.0	0.1	(0.3)
Oil and Condensate [Member] | As reported [Member] | Total [Member]
Revenue from Contract with Customer, Including Assessed Tax	285.7	408.5	535.2	709.2
Natural Gas [Member] | As reported [Member] | Williston Basin [Member]
Revenue from Contract with Customer, Including Assessed Tax	7.5	8.4	20.0	18.2
Natural Gas [Member] | As reported [Member] | Uinta Basin [Member]
Revenue from Contract with Customer, Including Assessed Tax		7.9		18.0
Natural Gas [Member] | As reported [Member] | Other Northern [Member]
Revenue from Contract with Customer, Including Assessed Tax	0.1	0.2	0.3	1.2
Natural Gas [Member] | As reported [Member] | Permian Basin [Member]
Revenue from Contract with Customer, Including Assessed Tax	(0.6)	3.2	4.0	7.8
Natural Gas [Member] | As reported [Member] | Haynesville/Cotton Valley [Member]
Revenue from Contract with Customer, Including Assessed Tax		77.9		154.3
Natural Gas [Member] | As reported [Member] | Other Southern [Member]
Revenue from Contract with Customer, Including Assessed Tax	0.3	0.2	6.0	0.3
Natural Gas [Member] | As reported [Member] | Total [Member]
Revenue from Contract with Customer, Including Assessed Tax	7.3	97.8	30.3	199.8
Natural Gas, Midstream [Member] | As reported [Member] | Williston Basin [Member]
Revenue from Contract with Customer, Including Assessed Tax	5.8	14.7	13.2	26.5
Natural Gas, Midstream [Member] | As reported [Member] | Uinta Basin [Member]
Revenue from Contract with Customer, Including Assessed Tax		1.7		3.4
Natural Gas, Midstream [Member] | As reported [Member] | Other Northern [Member]
Revenue from Contract with Customer, Including Assessed Tax	0.0	0.1	0.0	(0.1)
Natural Gas, Midstream [Member] | As reported [Member] | Permian Basin [Member]
Revenue from Contract with Customer, Including Assessed Tax	8.5	9.9	18.0	16.4
Natural Gas, Midstream [Member] | As reported [Member] | Haynesville/Cotton Valley [Member]
Revenue from Contract with Customer, Including Assessed Tax		0.0		0.0
Natural Gas, Midstream [Member] | As reported [Member] | Other Southern [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0	0.0	0.0
Natural Gas, Midstream [Member] | As reported [Member] | Total [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	14.3	26.4	31.2	46.2
Natural Gas, Gathering, Transportation, Marketing and Processing [Member] | As reported [Member] | Williston Basin [Member]
Cost of Goods and Services Sold	(8.9)	(10.7)	(19.0)	(20.6)
Natural Gas, Gathering, Transportation, Marketing and Processing [Member] | As reported [Member] | Uinta Basin [Member]
Cost of Goods and Services Sold		0.0		0.0
Natural Gas, Gathering, Transportation, Marketing and Processing [Member] | As reported [Member] | Other Northern [Member]
Cost of Goods and Services Sold	0.0	0.0	0.0	0.0
Natural Gas, Gathering, Transportation, Marketing and Processing [Member] | As reported [Member] | Permian Basin [Member]
Cost of Goods and Services Sold	(3.8)	(1.7)	(7.5)	(4.5)
Natural Gas, Gathering, Transportation, Marketing and Processing [Member] | As reported [Member] | Haynesville/Cotton Valley [Member]
Cost of Goods and Services Sold		0.0		0.0
Natural Gas, Gathering, Transportation, Marketing and Processing [Member] | As reported [Member] | Other Southern [Member]
Cost of Goods and Services Sold	0.0	0.0	0.0	0.0
Natural Gas, Gathering, Transportation, Marketing and Processing [Member] | As reported [Member] | Total [Member]
Cost of Goods and Services Sold	(12.7)	(12.4)	(26.5)	(25.1)
Oil and Gas, Exploration and Production [Member]
Revenue from Contract with Customer, Including Assessed Tax	294.6	520.3	570.2	930.1
Oil and Gas, Exploration and Production [Member] | As reported [Member] | Williston Basin [Member]
Revenue from Contract with Customer, Including Assessed Tax	111.9	220.0	231.6	392.2
Oil and Gas, Exploration and Production [Member] | As reported [Member] | Uinta Basin [Member]
Revenue from Contract with Customer, Including Assessed Tax		19.1		39.3
Oil and Gas, Exploration and Production [Member] | As reported [Member] | Other Northern [Member]
Revenue from Contract with Customer, Including Assessed Tax	0.6	1.2	1.2	3.9
Oil and Gas, Exploration and Production [Member] | As reported [Member] | Permian Basin [Member]
Revenue from Contract with Customer, Including Assessed Tax	181.7	201.7	331.3	339.8
Oil and Gas, Exploration and Production [Member] | As reported [Member] | Haynesville/Cotton Valley [Member]
Revenue from Contract with Customer, Including Assessed Tax		78.1		154.9
Oil and Gas, Exploration and Production [Member] | As reported [Member] | Other Southern [Member]
Revenue from Contract with Customer, Including Assessed Tax	0.4	0.2	6.1	0.0
Oil and Gas, Exploration and Production [Member] | As reported [Member] | Total [Member]
Revenue from Contract with Customer, Including Assessed Tax	$ 294.6	$ 520.3	$ 570.2	$ 930.1