Employee Benefit Plans (Schedule of Net Benefit Costs) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2016	Oct. 01, 2016	Dec. 31, 2017	Dec. 31, 2015
Successor
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1,583		$ 6,320
Interest cost	1,126		4,651
Successor | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	0		0
Interest cost	2,768		11,169
Curtailments	0		0
Settlements	(135)		(1,532)
Expected return on plan assets	(4,770)		(16,498)
Amortization of prior service credits	0		0
Amortization of other actuarial losses (gains)	0		0
Total occupational disease	(2,137)		(6,861)
Successor | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	180		671
Interest cost	978		4,150
Curtailments	0		(520)
Settlements	0		0
Expected return on plan assets	0		0
Amortization of prior service credits	0		0
Amortization of other actuarial losses (gains)	0		0
Total occupational disease	$ 1,158		$ 4,301
Predecessor
Defined Benefit Plan Disclosure [Line Items]
Service cost		$ 3,465		$ 4,282
Interest cost		3,184		3,944
Predecessor | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost		0		9
Interest cost		9,338		14,604
Curtailments		454		0
Settlements		0		2,656
Expected return on plan assets		(13,623)		(20,367)
Amortization of prior service credits		0		0
Amortization of other actuarial losses (gains)		3,973		8,850
Total occupational disease		142		5,752
Predecessor | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost		393		866
Interest cost		3,223		1,904
Curtailments		(970)		0
Settlements		0		0
Expected return on plan assets		0		0
Amortization of prior service credits		(7,854)		(8,335)
Amortization of other actuarial losses (gains)		(849)		(2,109)
Total occupational disease		$ (6,057)		$ (7,674)