Property, Plant, Equipment and Mine Development - Narrative (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment	$ 0	$ 1,418,100,000	$ 0	$ 1,418,100,000
At-risk assets	1,200,000,000		1,200,000,000
Other US Mining Operations
Impaired Long-Lived Assets Held and Used [Line Items]
At-risk assets	$ 0		$ 0
Property, plant, equipment, and mine development assets
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment		1,393,700,000		1,393,700,000
Operating lease right-of-use assets
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment		19,900,000		19,900,000
Contract-Based Intangible Assets
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment		$ 4,500,000		$ 4,500,000