Segment Information (Schedule of Operating Segment Results) (Details) - USD ($) $ in Thousands	3 Months Ended								9 Months Ended	12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Oct. 01, 2016	Dec. 31, 2015	Dec. 31, 2014
Successor [Member]
Segment Reporting Information [Line Items]
Revenues	$ 575,688
Adjusted EBITDAR	94,497
Depreciation, depletion and amortization	32,604
Accretion on asset retirement obligations	7,634
Total assets	2,136,597
Capital expenditures	15,214
Successor [Member] | Operating Segments [Member] | PRB [Member]
Segment Reporting Information [Line Items]
Revenues	275,703
Adjusted EBITDAR	55,765
Depreciation, depletion and amortization	9,949
Accretion on asset retirement obligations	5,049
Total assets	446,775
Capital expenditures	934
Successor [Member] | Operating Segments [Member] | APP [Member]
Segment Reporting Information [Line Items]
Revenues	200,377
Adjusted EBITDAR	30,819
Depreciation, depletion and amortization	18,287
Accretion on asset retirement obligations	528
Total assets	576,793
Capital expenditures	13,329
Successor [Member] | Operating Segments [Member] | Other Thermal [Member]
Segment Reporting Information [Line Items]
Revenues	97,382
Adjusted EBITDAR	31,159
Depreciation, depletion and amortization	3,911
Accretion on asset retirement obligations	540
Total assets	129,602
Capital expenditures	684
Successor [Member] | Corporate, Other and Elimination
Segment Reporting Information [Line Items]
Revenues	2,226
Adjusted EBITDAR	(23,246)
Depreciation, depletion and amortization	457
Accretion on asset retirement obligations	1,517
Total assets	983,427
Capital expenditures	$ 267
Predecessor [Member]
Segment Reporting Information [Line Items]
Revenues		$ 550,305	$ 420,298	$ 428,106	$ 563,249	$ 688,544	$ 644,462	$ 677,005	$ 1,398,709	$ 2,573,260	$ 2,937,119
Adjusted EBITDAR									87,303	283,797	313,052
Depreciation, depletion and amortization									191,581	379,345	418,748
Accretion on asset retirement obligations									24,321	33,680	32,909
Total assets					5,041,881				2,123,829	5,041,881	8,346,362
Capital expenditures									82,434	119,024	147,286
Predecessor [Member] | Operating Segments [Member] | PRB [Member]
Segment Reporting Information [Line Items]
Revenues									726,747	1,448,440	1,490,377
Adjusted EBITDAR									113,185	281,039	218,731
Depreciation, depletion and amortization									100,151	176,257	168,522
Accretion on asset retirement obligations									16,940	22,156	20,748
Total assets					1,648,916				456,711	1,648,916	1,762,326
Capital expenditures									612	21,228	21,399
Predecessor [Member] | Operating Segments [Member] | APP [Member]
Segment Reporting Information [Line Items]
Revenues									437,069	637,941	743,973
Adjusted EBITDAR									11,851	70,450	112,719
Depreciation, depletion and amortization									55,311	133,463	163,644
Accretion on asset retirement obligations									1,765	2,267	2,089
Total assets					772,439				619,154	772,439	2,339,739
Capital expenditures									17,296	24,787	46,771
Predecessor [Member] | Operating Segments [Member] | Other Thermal [Member]
Segment Reporting Information [Line Items]
Revenues									213,052	428,809	535,783
Adjusted EBITDAR									31,448	42,734	78,238
Depreciation, depletion and amortization									32,310	47,786	52,991
Accretion on asset retirement obligations									1,988	2,658	2,412
Total assets					366,610				131,173	366,610	406,296
Capital expenditures									3,895	11,277	14,843
Predecessor [Member] | Corporate, Other and Elimination
Segment Reporting Information [Line Items]
Revenues									21,841	58,070	166,986
Adjusted EBITDAR									(69,181)	(110,426)	(96,636)
Depreciation, depletion and amortization									3,809	21,839	33,591
Accretion on asset retirement obligations									3,628	6,599	7,660
Total assets					$ 2,253,916				916,791	2,253,916	3,838,001
Capital expenditures									$ 60,631	$ 61,732	$ 64,273