Asset Impairment (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment charges	$ 1,487,400,000	$ 270,200,000	$ 0
Asset impairment of property, plant and equipment and mine development assets	1,487,400,000	261,200,000	0
Impairment to unallocated coal reserves	69,300,000	69,200,000
Impairment of investment in equity security	0	9,000,000.0	$ 0
Assets subject to recoverability review	1,200,000,000
Powder River Basin Mining
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment charges	1,418,100,000
Asset impairment of property, plant and equipment and mine development assets	1,393,700,000
Operating lease right-of-use assets impairment	19,900,000
Impairment of contract-based intangible assets	$ 4,500,000
Other U.S. Thermal Mining
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment charges		$ 192,000,000.0