Other Events (Details) AUD in Millions, $ in Millions		3 Months Ended				6 Months Ended
	Jul. 07, 2016 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Sep. 30, 2016 AUD	Jun. 30, 2016 AUD
Other Commercial Events [Line Items]
Restructuring charges		$ 3.1	$ 2.5		$ 21.2	$ 15.2	$ 21.2
Proceeds from sale of nonstrategic assets			113.8			116.0	23.9
Restricted cash and investments, noncurrent		32.6	32.6			32.6
Restricted cash and cash equivalents		47.1	47.1			47.1
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 20.0
Non-debtor lease committments		240.0	$ 240.0			240.0
Nonstrategic Illinois mining tenement [Member] [Member]
Other Commercial Events [Line Items]
Proceeds from sale of nonstrategic assets		57.1
Gain recognized on sale		6.2				6.2
Nonstrategic Australian mining tenement [Member]
Other Commercial Events [Line Items]
Proceeds from sale of nonstrategic assets		48.1				64.1
Gain recognized on sale		$ 2.8				$ 2.8
New Mexico/Colorado mining tenement [Member] [Member]
Other Commercial Events [Line Items]
Proceeds from sale of nonstrategic assets				$ 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%
Scenario, Forecast [Member]
Other Commercial Events [Line Items]
Cash Collateral | AUD								AUD 50.0
Scenario, Forecast [Member] | Nonstrategic Australian mining tenement [Member]
Other Commercial Events [Line Items]
Proceeds from sale of nonstrategic assets	$ 16.0