Revenue Disaggregated Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Transportation and processing costs	$ 34.0	$ 70.2
Oil and condensate, gas and NGL sales	409.8	385.2
As reported [Member]
Oil and condensate sales	300.7	221.7
Gas sales	102.0	134.5
NGL sales	19.8	29.0
Transportation and processing costs	34.0
Oil and condensate, gas and NGL sales	409.8	385.2
As reported [Member] | Williston Basin [Member]
Oil and condensate sales	160.5	155.4
Gas sales	9.8	12.0
NGL sales	11.8	12.5
Oil and condensate, gas and NGL sales	172.2	179.9
As reported [Member] | Pinedale [Member]
Oil and condensate sales		6.8
Gas sales		60.6
NGL sales		11.6
Oil and condensate, gas and NGL sales		79.0
As reported [Member] | Uinta Basin [Member]
Oil and condensate sales	8.4	7.4
Gas sales	10.1	14.6
NGL sales	1.7	1.6
Oil and condensate, gas and NGL sales	20.2	23.6
As reported [Member] | Other Northern [Member]
Oil and condensate sales	1.9	1.4
Gas sales	1.0	5.9
NGL sales	(0.2)	0.1
Oil and condensate, gas and NGL sales	2.7	7.4
As reported [Member] | Permian Basin [Member]
Oil and condensate sales	129.8	50.2
Gas sales	4.6	3.2
NGL sales	6.5	3.1
Oil and condensate, gas and NGL sales	138.1	56.5
As reported [Member] | Haynesville/Cotton Valley [Member]
Oil and condensate sales	0.4	0.4
Gas sales	76.4	38.2
NGL sales	0.0	0.1
Oil and condensate, gas and NGL sales	76.8	38.7
As reported [Member] | Other Southern [Member]
Oil and condensate sales	(0.3)	0.1
Gas sales	0.1	0.0
NGL sales	0.0	0.0
Oil and condensate, gas and NGL sales	(0.2)	0.1
Revenues as reported [Member]
Transportation and processing costs	(12.7)	0.0
Revenues as reported [Member] | Williston Basin [Member]
Transportation and processing costs	(9.9)	0.0
Revenues as reported [Member] | Pinedale [Member]
Transportation and processing costs		0.0
Revenues as reported [Member] | Uinta Basin [Member]
Transportation and processing costs	0.0	0.0
Revenues as reported [Member] | Other Northern [Member]
Transportation and processing costs	0.0	0.0
Revenues as reported [Member] | Permian Basin [Member]
Transportation and processing costs	(2.8)	0.0
Revenues as reported [Member] | Haynesville/Cotton Valley [Member]
Transportation and processing costs	0.0	0.0
Revenues as reported [Member] | Other Southern [Member]
Transportation and processing costs	$ 0.0	$ 0.0