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, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1,583		$ 7,440	$ 6,320
Interest cost	1,126		4,365	4,651
Amortization of other actuarial losses (gains)	0		0	0
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	0		0	0
Interest cost	2,768		9,269	11,169
Curtailments	0		0	0
Settlements	(135)		(2,332)	(1,532)
Expected return on plan assets	(4,770)		(12,083)	(16,498)
Amortization of prior service credits	0		0	0
Amortization of other actuarial losses (gains)	0		0	0
Total occupational disease	(2,137)		(5,146)	(6,861)
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	180		558	671
Interest cost	978		3,674	4,150
Curtailments	0		0	(520)
Settlements	0		0	0
Expected return on plan assets	0		0	0
Amortization of prior service credits	0		0	0
Amortization of other actuarial losses (gains)	0		0	0
Total occupational disease	$ 1,158		$ 4,232	$ 4,301
Predecessor
Defined Benefit Plan Disclosure [Line Items]
Service cost		$ 3,465
Interest cost		3,184
Amortization of other actuarial losses (gains)		(4,325)
Predecessor | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost		0
Interest cost		9,338
Curtailments		454
Settlements		0
Expected return on plan assets		(13,623)
Amortization of prior service credits		0
Amortization of other actuarial losses (gains)		3,973
Total occupational disease		142
Predecessor | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost		393
Interest cost		3,223
Curtailments		(970)
Settlements		0
Expected return on plan assets		0
Amortization of prior service credits		(7,854)
Amortization of other actuarial losses (gains)		(849)
Total occupational disease		$ (6,057)