Other Events (Details) AUD in Millions, $ in Millions	3 Months Ended			6 Months Ended	9 Months Ended
	Sep. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 AUD
Other Commercial Events [Line Items]
Restructuring charges	$ 0.3		$ 1.8	$ 2.8	$ 15.5	$ 23.0
Proceeds from disposal of assets, net of notes receivable				132.5	134.7	39.6
Restricted cash and investments, noncurrent	70.6			70.6	70.6
Restricted Cash and Cash Equivalents, Noncurrent	1.4			1.4	1.4
Restricted cash and cash equivalents	22.8			22.8	22.8
Reduction in estimated take-or-pay obligations	45.0			45.0	45.0
Take-or-pay obligation settlement					15.5	$ 0.0
Cash Collateral | AUD							AUD 50.0
Non-debtor lease committments	215.0			$ 215.0	215.0
Nonstrategic Illinois mining tenement [Member] [Member]
Other Commercial Events [Line Items]
Proceeds from disposal of assets, net of notes receivable	57.1
Gain recognized on sale	6.2				6.2
Nonstrategic Australian mining tenement [Member]
Other Commercial Events [Line Items]
Proceeds from disposal of assets, net of notes receivable					64.1
Gain recognized on sale	$ 2.8				$ 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
Prarie State Energy Campus [Member]
Other Commercial Events [Line Items]
Undivided Interest Percent Of New Electricity Generation Project	5.06%			5.06%	5.06%		5.06%