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Supplemental Oil and Gas Information (Unaudited) - Costs Incurred in Oil and Gas Property Acquisition, Exploration and Development Activities (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Property acquisition costs |  |  |
Unproved | $ 14.3 | $ 547.0 | $ 3.0
Proved | 0.2 | 733.1 | 794.3
Total acquisition costs | 14.5 | 1,280.1 | 797.3
Cost Incurred, Refunded Exploration Costs |  |  | 126.0
Exploration costs | 58.6 | 118.6 |
Development costs | 736.8 | 1,282.4 | 1,390.0
Total costs incurred | 809.9 | 2,681.1 | 2,313.3
Charged to expense |  |  |
Dry hole expense |  |  | 20.6
Geophysical and other costs | 38.6 | 54.3 | 47.4
Total charged to expense | 38.6 | 54.3 | 68.0
Property additions | 771.3 | 2,626.8 | 2,245.3
U.S. |  |  |
Property acquisition costs |  |  |
Unproved | 6.5 | 533.8 | 2.8
Proved | 0.2 | 733.1 | 794.3
Total acquisition costs | 6.7 | 1,266.9 | 797.1
Cost Incurred, Refunded Exploration Costs |  |  | 88.1
Exploration costs | 34.3 | 44.8 |
Development costs | 609.2 | 979.0 | 853.7
Total costs incurred | 650.2 | 2,290.7 | 1,738.9
Charged to expense |  |  |
Dry hole expense |  |  | 16.0
Geophysical and other costs | 14.3 | 21.6 | 13.4
Total charged to expense | 14.3 | 21.6 | 29.4
Property additions | 635.9 | 2,269.1 | 1,709.5
Canada |  |  |
Property acquisition costs |  |  |
Unproved | 0.5 | 0.2 | 0.0
Proved | 0.0 | 0.0 | 0.0
Total acquisition costs | 0.5 | 0.2 | 0.0
Cost Incurred, Refunded Exploration Costs |  |  | 0.6
Exploration costs | (0.4) | 6.4 |
Development costs | 120.8 | 281.8 | 373.8
Total costs incurred | 120.9 | 288.4 | 374.4
Charged to expense |  |  |
Dry hole expense |  |  | 0.0
Geophysical and other costs | 0.7 | 0.5 | 0.6
Total charged to expense | 0.7 | 0.5 | 0.6
Property additions | 120.2 | 287.9 | 373.8
Malaysia |  |  |
Property acquisition costs |  |  |
Unproved | 0.0 | 0.0 | 0.0
Proved | 0.0 | 0.0 | 0.0
Total acquisition costs | 0.0 | 0.0 | 0.0
Cost Incurred, Refunded Exploration Costs |  |  | 2.2
Exploration costs | 0.0 | 0.0 |
Development costs | 0.0 | 0.0 | 145.9
Total costs incurred | 0.0 | 0.0 | 148.1
Charged to expense |  |  |
Dry hole expense |  |  | 0.1
Geophysical and other costs | 0.0 | 0.0 | 2.1
Total charged to expense | 0.0 | 0.0 | 2.2
Property additions | 0.0 | 0.0 | 145.9
Other |  |  |
Property acquisition costs |  |  |
Unproved | 7.3 | 13.0 | 0.2
Proved | 0.0 | 0.0 | 0.0
Total acquisition costs | 7.3 | 13.0 | 0.2
Cost Incurred, Refunded Exploration Costs |  |  | 35.1
Exploration costs | 24.7 | 67.4 |
Development costs | 6.8 | 21.6 | 16.6
Total costs incurred | 38.8 | 102.0 | 51.9
Charged to expense |  |  |
Dry hole expense |  |  | 4.5
Geophysical and other costs | 23.6 | 32.2 | 31.3
Total charged to expense | 23.6 | 32.2 | 35.8
Property additions | 15.2 | 69.8 | 16.1
Asset retirement obligations |  |  |
Property acquisition costs |  |  |
Exploration costs | 0.0 | 0.0 | 0.0
Development costs | 14.7 | 79.6 | 373.5
Total costs incurred | 14.7 | 79.6 | 373.5
Asset retirement obligations | U.S. |  |  |
Property acquisition costs |  |  |
Exploration costs | 0.0 | 0.0 | 0.0
Development costs | 12.8 | 75.8 | 366.0
Total costs incurred | 12.8 | 75.8 | 366.0
Asset retirement obligations | Canada |  |  |
Property acquisition costs |  |  |
Exploration costs | 0.0 | 0.0 | 0.0
Development costs | 1.9 | 3.8 | 0.0
Total costs incurred | 1.9 | 3.8 | 0.0
Asset retirement obligations | Malaysia |  |  |
Property acquisition costs |  |  |
Exploration costs | 0.0 | 0.0 | 0.0
Development costs | 0.0 | 0.0 | 7.3
Total costs incurred | 0.0 | 0.0 | 7.3
Asset retirement obligations | Other |  |  |
Property acquisition costs |  |  |
Exploration costs | 0.0 | 0.0 | 0.0
Development costs | 0.0 | 0.0 | 0.2
Total costs incurred | $ 0.0 | $ 0.0 | $ 0.2

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