Revenue Disaggregated Revenue (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |
Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2019 | Sep. 30, 2018 |
Oil and Condensate [Member] | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contract with Customer, Excluding Assessed Tax | $ 298.8 | $ 416.1 | $ 834 | $ 1,125.3 |
Oil and Condensate [Member] | Williston Basin [Member] | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contract with Customer, Excluding Assessed Tax | 88.9 | 203.4 | 306.3 | 571.5 |
Oil and Condensate [Member] | Uinta Basin [Member] | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contract with Customer, Excluding Assessed Tax | | 7.3 | | 25.2 |
Oil and Condensate [Member] | Other Northern [Member] | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contract with Customer, Excluding Assessed Tax | 0 | 1.1 | 0.9 | 3.9 |
Oil and Condensate [Member] | Permian Basin [Member] | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contract with Customer, Excluding Assessed Tax | 209.9 | 204 | 526.7 | 524.1 |
Oil and Condensate [Member] | Haynesville [Member] | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contract with Customer, Excluding Assessed Tax | | 0.2 | | 0.8 |
Oil and Condensate [Member] | Other Southern [Member] | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contract with Customer, Excluding Assessed Tax | 0 | 0.1 | 0.1 | (0.2) |
Natural Gas [Member] | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contract with Customer, Excluding Assessed Tax | 9.1 | 101.7 | 39.4 | 301.5 |
Natural Gas [Member] | Williston Basin [Member] | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contract with Customer, Excluding Assessed Tax | 5.5 | 12 | 25.5 | 30.2 |
Natural Gas [Member] | Uinta Basin [Member] | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contract with Customer, Excluding Assessed Tax | | 6.8 | | 24.8 |
Natural Gas [Member] | Other Northern [Member] | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contract with Customer, Excluding Assessed Tax | 0.1 | 0.5 | 0.4 | 1.7 |
Natural Gas [Member] | Permian Basin [Member] | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contract with Customer, Excluding Assessed Tax | 3.4 | 5.4 | 7.4 | 13.2 |
Natural Gas [Member] | Haynesville [Member] | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contract with Customer, Excluding Assessed Tax | | 76.9 | | 231.2 |
Natural Gas [Member] | Other Southern [Member] | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contract with Customer, Excluding Assessed Tax | 0.1 | 0.1 | 6.1 | 0.4 |
Natural Gas Liquids [Member] | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contract with Customer, Excluding Assessed Tax | 11.9 | 42 | 43.1 | 88.2 |
Natural Gas Liquids [Member] | Williston Basin [Member] | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contract with Customer, Excluding Assessed Tax | 2.4 | 19.2 | 15.6 | 45.7 |
Natural Gas Liquids [Member] | Uinta Basin [Member] | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contract with Customer, Excluding Assessed Tax | | 1.4 | | 4.8 |
Natural Gas Liquids [Member] | Other Northern [Member] | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contract with Customer, Excluding Assessed Tax | 0.1 | 0.1 | 0.1 | 0 |
Natural Gas Liquids [Member] | Permian Basin [Member] | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contract with Customer, Excluding Assessed Tax | 9.4 | 21.3 | 27.4 | 37.7 |
Natural Gas Liquids [Member] | Haynesville [Member] | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contract with Customer, Excluding Assessed Tax | | 0 | | 0 |
Natural Gas Liquids [Member] | Other Southern [Member] | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contract with Customer, Excluding Assessed Tax | 0 | 0 | 0 | 0 |
Natural Gas, Gathering, Transportation, Marketing and Processing [Member] | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Cost of Goods and Services Sold | (14.2) | (15.8) | (40.7) | (40.9) |
Natural Gas, Gathering, Transportation, Marketing and Processing [Member] | Williston Basin [Member] | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Cost of Goods and Services Sold | (7) | (12.3) | (26) | (32.9) |
Natural Gas, Gathering, Transportation, Marketing and Processing [Member] | Uinta Basin [Member] | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Cost of Goods and Services Sold | | 0 | | 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 |
Natural Gas, Gathering, Transportation, Marketing and Processing [Member] | Permian Basin [Member] | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Cost of Goods and Services Sold | (7.2) | (3.5) | (14.7) | (8) |
Natural Gas, Gathering, Transportation, Marketing and Processing [Member] | Haynesville [Member] | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Cost of Goods and Services Sold | | 0 | | 0 |
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 |
Oil and Gas, Exploration and Production [Member] | As reported [Member] | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contract with Customer, Excluding Assessed Tax | 305.6 | 544 | 875.8 | 1,474.1 |
Oil and Gas, Exploration and Production [Member] | Williston Basin [Member] | As reported [Member] | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contract with Customer, Excluding Assessed Tax | 89.8 | 222.3 | 321.4 | 614.5 |
Oil and Gas, Exploration and Production [Member] | Uinta Basin [Member] | As reported [Member] | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contract with Customer, Excluding Assessed Tax | | 15.5 | | 54.8 |
Oil and Gas, Exploration and Production [Member] | Other Northern [Member] | As reported [Member] | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contract with Customer, Excluding Assessed Tax | 0.2 | 1.7 | 1.4 | 5.6 |
Oil and Gas, Exploration and Production [Member] | Permian Basin [Member] | As reported [Member] | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contract with Customer, Excluding Assessed Tax | 215.5 | 227.2 | 546.8 | 567 |
Oil and Gas, Exploration and Production [Member] | Haynesville [Member] | As reported [Member] | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contract with Customer, Excluding Assessed Tax | | 77.1 | | 232 |
Oil and Gas, Exploration and Production [Member] | Other Southern [Member] | As reported [Member] | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contract with Customer, Excluding Assessed Tax | $ 0.1 | $ 0.2 | $ 6.2 | $ 0.2 |