Asset Impairment	6 Months Ended
Jun. 30, 2017
Asset Impairment Charges [Abstract]
Asset Impairment	Asset Impairment The Company's mining and exploration assets and mining-related investments may be adversely affected by numerous factors that may cause the Company to be unable to recover all or a portion of the carrying value of those assets. As a result of various unfavorable conditions, including but not limited to sustained trends of weakness in U.S. and international seaborne coal market pricing and certain asset-specific factors, the Company recognized aggregate impairment charges of $247.9 million, $1,277.8 million, and $154.4 million during the years ended December 31, 2016, 2015 and 2014, respectively. For additional information surrounding those charges, refer to Note 4. "Asset Impairment" to the consolidated financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2016, as amended. The Company generally does not view short-term declines subsequent to previous impairment assessments in thermal and metallurgical coal prices in the markets in which it sells its products as an indicator of impairment. However, the Company generally views a sustained trend (for example, over periods exceeding one year) of adverse coal market pricing or unfavorable changes thereto as a potential indicator of impairment. Because of the volatile and cyclical nature of U.S. and international seaborne coal markets, it is reasonably possible that prices in those market segments may decrease and/or fail to improve in the near term, which, absent sufficient mitigation such as an offsetting reduction in the Company's operating costs, may result in the need for future adjustments to the carrying value of the Company's long-lived mining assets and mining-related investments. During the period January 1 through April 1, 2017, the Company recognized impairment charges of $30.5 million related to terminated coal lease contracts in the Midwestern United States and during the six months ended June 30, 2016 the Company recognized impairment charges of $17.2 million to write down certain targeted divestiture assets in Queensland, Australia.

Asset Impairment

The Company's mining and exploration assets and mining-related investments may be adversely affected by numerous factors that may cause the Company to be unable to recover all or a portion of the carrying value of those assets. As a result of various unfavorable conditions, including but not limited to sustained trends of weakness in U.S. and international seaborne coal market pricing and certain asset-specific factors, the Company recognized aggregate impairment charges of $247.9 million, $1,277.8 million, and $154.4 million during the years ended December 31, 2016, 2015 and 2014, respectively. For additional information surrounding those charges, refer to Note 4. "Asset Impairment" to the consolidated financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2016, as amended.

The Company generally does not view short-term declines subsequent to previous impairment assessments in thermal and metallurgical coal prices in the markets in which it sells its products as an indicator of impairment. However, the Company generally views a sustained trend (for example, over periods exceeding one year) of adverse coal market pricing or unfavorable changes thereto as a potential indicator of impairment. Because of the volatile and cyclical nature of U.S. and international seaborne coal markets, it is reasonably possible that prices in those market segments may decrease and/or fail to improve in the near term, which, absent sufficient mitigation such as an offsetting reduction in the Company's operating costs, may result in the need for future adjustments to the carrying value of the Company's long-lived mining assets and mining-related investments.

During the period January 1 through April 1, 2017, the Company recognized impairment charges of $30.5 million related to terminated coal lease contracts in the Midwestern United States and during the six months ended June 30, 2016 the Company recognized impairment charges of $17.2 million to write down certain targeted divestiture assets in Queensland, Australia.