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Supplemental Oil and Gas Information (Costs Incurred in Oil and Gas Property Acquisition, Exploration and Development Activities) (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Property acquisition costs |  |  |
Unproved | $ 3.0 | $ 63.4 | $ 18.6
Proved | 794.3 | 7.7 | 206.7
Total acquisition costs | 797.3 | 71.1 | 225.3
Exploration costs | 126.0 | 79.2 | 70.1
Development costs | 1,390.0 | 819.0 | 508.0
Total costs incurred | 2,313.3 | 969.3 | 803.4
Charged to expense |  |  |
Dry hole expense | 20.6 | (4.2) | 15.1
Geophysical and other costs | 47.4 | 65.2 | 43.4
Total charged to expense | 68.0 | 61.0 | 58.5
Property additions | 2,245.3 | 908.3 | 744.9
United States [Member] |  |  |
Property acquisition costs |  |  |
Unproved | 2.8 | 50.4 | 18.6
Proved | 794.3 | 7.7 |
Total acquisition costs | 797.1 | 58.1 | 18.6
Exploration costs | 88.1 | 13.7 | 18.5
Development costs | 853.7 | 508.4 | 239.7
Total costs incurred | 1,738.9 | 580.2 | 276.8
Charged to expense |  |  |
Dry hole expense | 16.0 | (1.9) | 0.4
Geophysical and other costs | 13.4 | 9.7 | 5.7
Total charged to expense | 29.4 | 7.8 | 6.1
Property additions | 1,709.5 | 572.4 | 270.7
Canada [Member] |  |  |
Property acquisition costs |  |  |
Proved |  |  | 206.7
Total acquisition costs |  |  | 206.7
Exploration costs | 0.6 | 0.6 | 3.6
Development costs | 373.8 | 273.8 | 165.1
Total costs incurred | 374.4 | 274.4 | 375.4
Charged to expense |  |  |
Geophysical and other costs | 0.6 | 0.5 | 3.6
Total charged to expense | 0.6 | 0.5 | 3.6
Property additions | 373.8 | 273.9 | 371.8
Malaysia [Member] |  |  |
Property acquisition costs |  |  |
Exploration costs | 2.2 | (8.9) | 6.0
Development costs | 145.9 | 35.7 | 102.9
Total costs incurred | 148.1 | 26.8 | 108.9
Charged to expense |  |  |
Dry hole expense | 0.1 | 0.7 | 4.5
Geophysical and other costs | 2.1 | 1.7 | 0.7
Total charged to expense | 2.2 | 2.4 | 5.2
Property additions | 145.9 | 24.4 | 103.7
Other Regions [Member] |  |  |
Property acquisition costs |  |  |
Unproved | 0.2 | 13.0 |
Total acquisition costs | 0.2 | 13.0 |
Exploration costs | 35.1 | 73.8 | 42.0
Development costs | 16.6 | 1.1 | 0.3
Total costs incurred | 51.9 | 87.9 | 42.3
Charged to expense |  |  |
Dry hole expense | 4.5 | (3.0) | 10.2
Geophysical and other costs | 31.3 | 53.3 | 33.4
Total charged to expense | 35.8 | 50.3 | 43.6
Property additions | 16.1 | 37.6 | (1.3)
Asset Retirement Obligation Costs [Member] |  |  |
Property acquisition costs |  |  |
Development costs | 373.5 | 52.3 | 13.7
Total costs incurred | 373.5 | 52.3 | 13.7
Asset Retirement Obligation Costs [Member] | United States [Member] |  |  |
Property acquisition costs |  |  |
Development costs | 366.0 | 37.6 | 0.9
Total costs incurred | 366.0 | 37.6 | 0.9
Asset Retirement Obligation Costs [Member] | Canada [Member] |  |  |
Property acquisition costs |  |  |
Development costs |  | 6.3 | 10.5
Total costs incurred |  | 6.3 | 10.5
Asset Retirement Obligation Costs [Member] | Malaysia [Member] |  |  |
Property acquisition costs |  |  |
Development costs | 7.3 | 8.4 | 2.3
Total costs incurred | 7.3 | $ 8.4 | $ 2.3
Asset Retirement Obligation Costs [Member] | Other Regions [Member] |  |  |
Property acquisition costs |  |  |
Development costs | 0.2 |  |
Total costs incurred | $ 0.2 |  |

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