Property, Plant, Equipment and Mine Development - Narrative (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment	$ 0	$ 20,000,000.0	$ 1,418,100,000	$ 20,000,000.0
At-risk assets	1,100,000,000		1,100,000,000
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
Operating lease right-of-use assets
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment			19,900,000
Contract-Based Intangible Assets
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment			$ 4,500,000