Revenue Disaggregated Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Transportation and processing costs	$ 10.9	$ 34.0
Oil and condensate, gas and NGL sales	275.6	409.8
As reported [Member]
Oil and condensate sales	249.5	300.7
Gas sales	23.0	102.0
NGL sales	16.9	19.8
Oil and condensate, gas and NGL sales	275.6	409.8
As reported [Member] | Williston Basin [Member]
Oil and condensate sales	109.9	160.5
Gas sales	12.5	9.8
NGL sales	7.4	11.8
Transportation and processing costs	(10.1)	(9.9)
Oil and condensate, gas and NGL sales	119.7	172.2
As reported [Member] | Uinta Basin [Member]
Oil and condensate sales		8.4
Gas sales		10.1
NGL sales		1.7
Transportation and processing costs		0.0
Oil and condensate, gas and NGL sales		20.2
As reported [Member] | Other Northern [Member]
Oil and condensate sales	0.4	1.9
Gas sales	0.2	1.0
NGL sales	0.0	(0.2)
Transportation and processing costs	0.0	0.0
Oil and condensate, gas and NGL sales	0.6	2.7
As reported [Member] | Permian Basin [Member]
Oil and condensate sales	139.2	129.8
Gas sales	4.6	4.6
NGL sales	9.5	6.5
Transportation and processing costs	(3.7)	(2.8)
Oil and condensate, gas and NGL sales	149.6	138.1
As reported [Member] | Haynesville/Cotton Valley [Member]
Oil and condensate sales		0.4
Gas sales		76.4
NGL sales		0.0
Transportation and processing costs		0.0
Oil and condensate, gas and NGL sales		76.8
As reported [Member] | Other Southern [Member]
Oil and condensate sales	0.0	(0.3)
Gas sales	5.7	0.1
NGL sales	0.0	0.0
Transportation and processing costs	0.0	0.0
Oil and condensate, gas and NGL sales	5.7	(0.2)
As reported [Member] | Total [Member]
Transportation and processing costs	$ (13.8)	$ (12.7)