Other Events (Details) - USD ($) $ in Millions	3 Months Ended			12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Sep. 30, 2016
Other Commercial Events [Line Items]
Restructuring charges	$ 0.0	$ 12.1
Restricted Cash and Investments, Current	80.7			$ 54.3
Proceeds from disposal of assets, net of notes receivable	24.3	2.1
Restricted Cash and Investments, Noncurrent	88.0			71.4
Restricted cash and cash equivalents	66.9
Take-or-pay obligation settlement	5.5	15.5
Asset Impairment Charges	30.5	17.2		247.9	$ 1,277.8	$ 154.4
Net gain on disposal of assets	22.8	$ 1.8
Cash Collateral	89.2
Nonstrategic Illinois mining tenement [Member] [Member]
Other Commercial Events [Line Items]
Gain recognized on sale	6.2
Nonstrategic Australian mining tenement [Member]
Other Commercial Events [Line Items]
Gain recognized on sale	2.8
New Mexico/Colorado mining tenement [Member] [Member]
Other Commercial Events [Line Items]
Proceeds from disposal of assets, net of notes receivable					$ 358.0
Termination fee			$ 20.0
DTA and PBGC [Member]
Other Commercial Events [Line Items]
Proceeds from disposal of assets, net of notes receivable	20.5
Net gain on disposal of assets	$ (19.7)
Metropolitan mine [Member]
Other Commercial Events [Line Items]
Cash consideration for asset held for sale							$ 200.0
Asset Impairment Charges				$ 193.2
Dominion Terminal Associates Partnership [Member]
Other Commercial Events [Line Items]
Equity Method Investment, Ownership Percentage				37.50%