Other Events (Details) - USD ($) shares in Millions, $ in Millions		1 Months Ended		3 Months Ended			6 Months Ended	9 Months Ended
	Aug. 23, 2017	Oct. 30, 2017	Sep. 30, 2017	Sep. 30, 2017	Apr. 01, 2017	Mar. 31, 2017	Sep. 30, 2017	Sep. 30, 2016	Sep. 05, 2017	Aug. 29, 2017	Aug. 01, 2017	Dec. 31, 2016	Nov. 30, 2016	Jun. 30, 2016
Other Commercial Events [Line Items]
Amount available for repurchase of shares under share repurchase program											$ 500.0
DTA and PBGC
Other Commercial Events [Line Items]
Equity method investment, ownership percentage												37.50%
Proceeds from disposal of assets, net of notes receivable						$ 20.5
Net gain on disposal of assets					$ 19.7
Metropolitan Collieries Pty Ltd
Other Commercial Events [Line Items]
Planned divestiture, agreed upon proceeds from divestiture													$ 200.0
Lenton Joint Venture [Member] | Burton Mine [Member]
Other Commercial Events [Line Items]
Planned divestiture, agreed upon proceeds from divestiture									$ 11.0
Increase (Decrease) in Asset Retirement Obligations				$ 53.0
Increase (Decrease) in Restricted Cash				30.0
BHP Billiton Mitsui Coal Pty Ltd [Member] | Red Mountain Joint Venture [Member]
Other Commercial Events [Line Items]
Planned divestiture, agreed upon proceeds from divestiture										$ 20.0
Ownership Percentage In Subsidiaries										50.00%
Bowie Resource Partners, LLC | New Mexico and Colorado Assets
Other Commercial Events [Line Items]
Planned divestiture, agreed upon proceeds from divestiture													$ 358.0
Termination fee							$ 20.0
Wabash Valley Power Association [Member] | Prarie State Energy Campus [Member]
Other Commercial Events [Line Items]
Proceeds from disposal of assets, net of notes receivable								$ 57.1
Net gain on disposal of assets								6.2
Undivided Interest Percent Of New Electricity Generation Project														5.10%
Pembroke Resources [Member] | NonstrategicAustralianminingtenement [Member]
Other Commercial Events [Line Items]
Proceeds from disposal of assets, net of notes receivable								64.1
Net gain on disposal of assets								$ 2.8
Subsequent Event
Other Commercial Events [Line Items]
Treasury Stock, Shares, Acquired		1.3
Common stock repurchases		$ 37.7
Successor
Other Commercial Events [Line Items]
Proceeds from disposal of assets, net of notes receivable							5.2
Net gain on disposal of assets				$ 0.4			0.9
Increase (Decrease) in Asset Retirement Obligations							$ 7.6
Treasury Stock, Shares, Acquired	1.5		1.0				2.5
Common stock repurchases	$ 40.0		$ 29.2