Employee Benefit Plans - Schedule of Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Interest cost	$ 4,656	$ 6,677	$ 9,391	$ 13,293
Expected return on plan assets	(6,748)	(7,015)	(13,560)	(14,021)
Amortization of net actuarial loss (gain)	521	190	990	398
Settlement	63	0	1,230	0
Net periodic expense	(1,508)	(148)	(1,949)	(330)
Black Lung
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	585	403	1,164	807
Interest cost	848	909	1,746	1,819
Expected return on plan assets	(13)	(16)	(26)	(32)
Amortization of net actuarial loss (gain)	419	45	750	90
Net periodic expense	1,839	1,341	3,634	2,684
Life Insurance Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Interest cost	85	106	169	212
Amortization of net actuarial loss (gain)	(12)	(26)	(24)	(52)
Net periodic expense	$ 73	$ 80	$ 145	$ 160