Resource Management, Acquisitions and Other Commercial Events (Details) AUD in Millions, $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended	12 Months Ended
	Jan. 31, 2014 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2016 AUD
OtherCommercialEvents [Line Items]
Restructuring Charges				$ 2.8	$ 15.5	$ 23.5	$ 26.0
Held for sale, at carrying value			$ 125.0			125.0
Proceeds from disposal of assets, net of notes receivable				142.2	144.4	70.4	203.7
Liabilities			10,195.2	$ 11,596.2	$ 11,596.2	10,195.2
Gain (loss) on sale of nonstrategic asset		$ 6.2
Interest in unincorporated joint venture project				50.00%	50.00%
Cash Collateral | AUD								AUD 50.0
Priairie State Campus [Member]
OtherCommercialEvents [Line Items]
Proceeds from disposal of assets, net of notes receivable					$ 57.1
Gain (loss) on sale of nonstrategic asset					6.2
NewMexico/Coloradominingtenement [Member] [Member]
OtherCommercialEvents [Line Items]
Proceeds from disposal of assets, net of notes receivable						$ 358.0
Gain (Loss) on Contract Termination			$ 20.0
Nonstrategic Australian mining tenement [Member]
OtherCommercialEvents [Line Items]
Proceeds from disposal of assets, net of notes receivable	$ 62.6				64.1
Gain (loss) on sale of nonstrategic asset		$ 2.8			0.0
Olivedownssouthminingtenement [Member] [Domain]
OtherCommercialEvents [Line Items]
Gain (loss) on sale of nonstrategic asset					$ 2.8
Nonstrategic Kentucky Coal Reserves And Surface Lands [Member]
OtherCommercialEvents [Line Items]
Proceeds from disposal of assets, net of notes receivable							29.6
Gain (loss) on sale of nonstrategic asset							$ 13.6
Prarie State Energy Campus [Member]
OtherCommercialEvents [Line Items]
Undivided Interest Percent Of New Electricity Generation Project				5.06%	5.06%