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Asset Retirement Obligations (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Jun. 30, 2016 | Dec. 31, 2016 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Sep. 30, 2016
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward] |  |  |  |  |  |
Total asset retirement obligation |  | $ 758.8 | $ 758.8 | $ 712.1 | $ 752.5 |
Current portion |  | 41.0 | 41.0 | 25.5 |  |
Liabilities incurred or acquired |  |  | 0.0 | 1.3 |  |
Liabilities settled or disposed |  |  | (41.5) | (53.3) |  |
Accretion expense |  |  | 45.7 | 42.7 |  |
Revision to estimates |  |  | 42.5 | (31.1) |  |
Noncurrent obligation |  | 717.8 | 717.8 | 686.6 |  |
Balance at end of year — active locations |  | 651.1 | 651.1 | 656.8 |  |
Balance at end of year — closed or inactive locations |  | 107.7 | 107.7 | 55.3 |  |
Net gain on disposal of assets | $ (13.7) | 21.4 | (23.2) | $ (45.0) | $ (41.4) |
Credit adjusted, risk-free interest rates |  |  |  | 50.83% | 6.82% |
Surety bonds and bank guarantees outstanding to secure reclamation obligations or activities |  | 374.3 | 374.3 | $ 609.4 |  |
Amount of reclamation self-bonding in certain states in which the company qualifies |  | 1,094.2 | 1,094.2 | 1,430.8 |  |
Letters of credit in support of reclamation obligations or activities |  | 80.0 | 80.0 | $ 126.6 |  | $ 15.7
Cash collateral in support of reclamation obligations or activities |  | $ 233.2 | $ 233.2 |  |  |
U.S. Obligations [Member] |  |  |  |  |  |
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward] |  |  |  |  |  |
Credit adjusted, risk-free interest rates |  | 13.45% | 13.45% |  |  |
Australian Operations [Member] |  |  |  |  |  |
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward] |  |  |  |  |  |
Credit adjusted, risk-free interest rates |  | 4.92% | 4.92% |  |  |

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- Definition Amount recognized for the passage of time, typically for liabilities, that have been discounted to their net present values. Excludes accretion associated with asset retirement obligations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6392676&loc=d3e7480-110848 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 420 -SubTopic 10 -Section 35 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6394232&loc=d3e17558-110866 + Details
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- Definition The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
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- Definition Current portion of the carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
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- Definition Amount of increase (decrease) in the asset retirement obligation from changes in the amount or timing of the estimated cash flows associated with the settlement of the obligation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c)(4) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 35 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=6392603&loc=d3e7123-110846 + Details
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- Definition Noncurrent portion of the carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
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