Revenues - Revenue by Product Type and Market (Details) - USD ($) $ in Millions			3 Months Ended			6 Months Ended
		Apr. 01, 2017	Jun. 30, 2018	Jun. 30, 2017	Apr. 01, 2017	Jun. 30, 2018
Successor
Disaggregation of Revenue [Line Items]
Revenues			$ 1,309.4	$ 1,258.3		$ 2,772.1
Successor | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues			921.6	918.7		1,851.7
Successor | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues			417.2	287.8		882.5
Successor | Other
Disaggregation of Revenue [Line Items]
Revenues			(29.4)	51.8		37.9
Successor | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues			688.0	707.8		1,444.5
Successor | Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues			233.6	210.9		407.2
Successor | Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues			417.2	287.8		882.5
Successor | Operating Segments | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues			321.5	365.4		710.8
Successor | Operating Segments | Powder River Basin Mining | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues			321.5	361.1		710.7
Successor | Operating Segments | Powder River Basin Mining | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues			0.0	0.0		0.0
Successor | Operating Segments | Powder River Basin Mining | Other
Disaggregation of Revenue [Line Items]
Revenues			0.0	4.3		0.1
Successor | Operating Segments | Powder River Basin Mining | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues			321.5	361.1		710.7
Successor | Operating Segments | Powder River Basin Mining | Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues			0.0	0.0		0.0
Successor | Operating Segments | Powder River Basin Mining | Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues			0.0	0.0		0.0
Successor | Operating Segments | Midwestern U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues			197.5	194.9		399.2
Successor | Operating Segments | Midwestern U.S. Mining | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues			197.5	194.8		399.1
Successor | Operating Segments | Midwestern U.S. Mining | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues			0.0	0.0		0.0
Successor | Operating Segments | Midwestern U.S. Mining | Other
Disaggregation of Revenue [Line Items]
Revenues			0.0	0.1		0.1
Successor | Operating Segments | Midwestern U.S. Mining | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues			196.9	194.8		397.8
Successor | Operating Segments | Midwestern U.S. Mining | Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues			0.6	0.0		1.3
Successor | Operating Segments | Midwestern U.S. Mining | Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues			0.0	0.0		0.0
Successor | Operating Segments | Western U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues			139.6	125.4		283.3
Successor | Operating Segments | Western U.S. Mining | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues			135.4	123.8		273.7
Successor | Operating Segments | Western U.S. Mining | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues			0.0	0.0		0.0
Successor | Operating Segments | Western U.S. Mining | Other
Disaggregation of Revenue [Line Items]
Revenues			4.2	1.6		9.6
Successor | Operating Segments | Western U.S. Mining | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues			131.1	123.8		261.4
Successor | Operating Segments | Western U.S. Mining | Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues			4.3	0.0		12.3
Successor | Operating Segments | Western U.S. Mining | Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues			0.0	0.0		0.0
Successor | Operating Segments | Australian Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues			417.5	287.8		883.7
Successor | Operating Segments | Australian Metallurgical Mining | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues			0.0	0.0		0.0
Successor | Operating Segments | Australian Metallurgical Mining | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues			417.2	287.8		882.5
Successor | Operating Segments | Australian Metallurgical Mining | Other
Disaggregation of Revenue [Line Items]
Revenues			0.3	0.0		1.2
Successor | Operating Segments | Australian Metallurgical Mining | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues			0.0	0.0		0.0
Successor | Operating Segments | Australian Metallurgical Mining | Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues			0.0	0.0		0.0
Successor | Operating Segments | Australian Metallurgical Mining | Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues			417.2	287.8		882.5
Successor | Operating Segments | Australian Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues			267.4	239.2		468.8
Successor | Operating Segments | Australian Thermal Mining | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues			267.2	239.0		468.2
Successor | Operating Segments | Australian Thermal Mining | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues			0.0	0.0		0.0
Successor | Operating Segments | Australian Thermal Mining | Other
Disaggregation of Revenue [Line Items]
Revenues			0.2	0.2		0.6
Successor | Operating Segments | Australian Thermal Mining | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues			38.5	28.1		74.6
Successor | Operating Segments | Australian Thermal Mining | Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues			228.7	210.9		393.6
Successor | Operating Segments | Australian Thermal Mining | Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues			0.0	0.0		0.0
Successor | Corporate, Non-Segment | Trading and Brokerage
Disaggregation of Revenue [Line Items]
Revenues			10.0	5.2		30.1
Successor | Corporate, Non-Segment | Trading and Brokerage | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues			0.0	0.0		0.0
Successor | Corporate, Non-Segment | Trading and Brokerage | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues			0.0	0.0		0.0
Successor | Corporate, Non-Segment | Trading and Brokerage | Other
Disaggregation of Revenue [Line Items]
Revenues			10.0	5.2		30.1
Successor | Corporate, Non-Segment | Trading and Brokerage | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues			0.0	0.0		0.0
Successor | Corporate, Non-Segment | Trading and Brokerage | Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues			0.0	0.0		0.0
Successor | Corporate, Non-Segment | Trading and Brokerage | Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues			0.0	0.0		0.0
Successor | Corporate, Non-Segment | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues	[1]		(44.1)	40.4		(3.8)
Successor | Corporate, Non-Segment | Corporate and Other | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	[1]		0.0	0.0		0.0
Successor | Corporate, Non-Segment | Corporate and Other | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	[1]		0.0	0.0		0.0
Successor | Corporate, Non-Segment | Corporate and Other | Other
Disaggregation of Revenue [Line Items]
Revenues	[1]		(44.1)	40.4		(3.8)
Successor | Corporate, Non-Segment | Corporate and Other | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	[1]		0.0	0.0		0.0
Successor | Corporate, Non-Segment | Corporate and Other | Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	[1]		0.0	0.0		0.0
Successor | Corporate, Non-Segment | Corporate and Other | Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	[1]		$ 0.0	$ 0.0		$ 0.0
Predecessor
Disaggregation of Revenue [Line Items]
Revenues		$ 0.0			$ 1,326.2
Predecessor | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues					945.5
Predecessor | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues					324.6
Predecessor | Other
Disaggregation of Revenue [Line Items]
Revenues					56.1
Predecessor | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues					748.3
Predecessor | Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues					197.2
Predecessor | Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues					324.6
Predecessor | Operating Segments | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues					394.3
Predecessor | Operating Segments | Powder River Basin Mining | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues					394.3
Predecessor | Operating Segments | Powder River Basin Mining | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues					0.0
Predecessor | Operating Segments | Powder River Basin Mining | Other
Disaggregation of Revenue [Line Items]
Revenues					0.0
Predecessor | Operating Segments | Powder River Basin Mining | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues					394.3
Predecessor | Operating Segments | Powder River Basin Mining | Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues					0.0
Predecessor | Operating Segments | Powder River Basin Mining | Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues					0.0
Predecessor | Operating Segments | Midwestern U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues					193.2
Predecessor | Operating Segments | Midwestern U.S. Mining | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues					193.2
Predecessor | Operating Segments | Midwestern U.S. Mining | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues					0.0
Predecessor | Operating Segments | Midwestern U.S. Mining | Other
Disaggregation of Revenue [Line Items]
Revenues					0.0
Predecessor | Operating Segments | Midwestern U.S. Mining | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues					193.2
Predecessor | Operating Segments | Midwestern U.S. Mining | Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues					0.0
Predecessor | Operating Segments | Midwestern U.S. Mining | Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues					0.0
Predecessor | Operating Segments | Western U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues					149.7
Predecessor | Operating Segments | Western U.S. Mining | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues					133.5
Predecessor | Operating Segments | Western U.S. Mining | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues					0.0
Predecessor | Operating Segments | Western U.S. Mining | Other
Disaggregation of Revenue [Line Items]
Revenues					16.2
Predecessor | Operating Segments | Western U.S. Mining | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues					133.5
Predecessor | Operating Segments | Western U.S. Mining | Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues					0.0
Predecessor | Operating Segments | Western U.S. Mining | Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues					0.0
Predecessor | Operating Segments | Australian Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues					328.9
Predecessor | Operating Segments | Australian Metallurgical Mining | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues					0.0
Predecessor | Operating Segments | Australian Metallurgical Mining | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues					324.6
Predecessor | Operating Segments | Australian Metallurgical Mining | Other
Disaggregation of Revenue [Line Items]
Revenues					4.3
Predecessor | Operating Segments | Australian Metallurgical Mining | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues					0.0
Predecessor | Operating Segments | Australian Metallurgical Mining | Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues					0.0
Predecessor | Operating Segments | Australian Metallurgical Mining | Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues					324.6
Predecessor | Operating Segments | Australian Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues					224.8
Predecessor | Operating Segments | Australian Thermal Mining | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues					224.5
Predecessor | Operating Segments | Australian Thermal Mining | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues					0.0
Predecessor | Operating Segments | Australian Thermal Mining | Other
Disaggregation of Revenue [Line Items]
Revenues					0.3
Predecessor | Operating Segments | Australian Thermal Mining | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues					27.3
Predecessor | Operating Segments | Australian Thermal Mining | Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues					197.2
Predecessor | Operating Segments | Australian Thermal Mining | Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues					0.0
Predecessor | Corporate, Non-Segment | Trading and Brokerage
Disaggregation of Revenue [Line Items]
Revenues					15.0
Predecessor | Corporate, Non-Segment | Trading and Brokerage | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues					0.0
Predecessor | Corporate, Non-Segment | Trading and Brokerage | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues					0.0
Predecessor | Corporate, Non-Segment | Trading and Brokerage | Other
Disaggregation of Revenue [Line Items]
Revenues					15.0
Predecessor | Corporate, Non-Segment | Trading and Brokerage | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues					0.0
Predecessor | Corporate, Non-Segment | Trading and Brokerage | Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues					0.0
Predecessor | Corporate, Non-Segment | Trading and Brokerage | Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues					0.0
Predecessor | Corporate, Non-Segment | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues	[1]				20.3
Predecessor | Corporate, Non-Segment | Corporate and Other | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	[1]				0.0
Predecessor | Corporate, Non-Segment | Corporate and Other | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	[1]				0.0
Predecessor | Corporate, Non-Segment | Corporate and Other | Other
Disaggregation of Revenue [Line Items]
Revenues	[1]				20.3
Predecessor | Corporate, Non-Segment | Corporate and Other | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	[1]				0.0
Predecessor | Corporate, Non-Segment | Corporate and Other | Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	[1]				0.0
Predecessor | Corporate, Non-Segment | Corporate and Other | Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	[1]				$ 0.0

[1] Corporate and Other revenue includes unrealized gains and losses related to mark-to-market activity from financial contract hedge activities intended to secure pricing related to certain coal sales contracts. During the three and six months ended June 30, 2018, such net unrealized losses were $48.1 million and $9.5 million, respectively. During the periods April 2 through June 30, 2017 and January 1 through April 1, 2017, such net unrealized gains were $9.4 million and $16.6 million, respectively. When such gains and losses are realized in connection with recognition of the underlying transaction, they are reclassified to realized gains and losses and are then reflected in Trading and Brokerage revenue (realized losses of $9.6 million and $29.5 million during the three and six months ended June 30, 2018, respectively, and realized losses of $7.7 million and $11.1 million during the periods April 2 through June 30, 2017 and January 1 through April 1, 2017, respectively). At June 30, 2018 and December 31, 2017, the financial contracts’ fair values resulted in net liabilities of $48.4 million and $38.9 million, respectively. Assets and liabilities related to the financial contracts were deemed Level 2 according to the U.S. GAAP fair value hierarchy as the valuations are corroborated by the use of market-based pricing.