Employee Benefit Plans - Schedule of Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Interest cost	$ 4,670	$ 6,636	$ 14,061	$ 19,929
Expected return on plan assets	(6,752)	(7,011)	(20,312)	(21,032)
Amortization of net actuarial loss (gain)	511	199	1,501	597
Settlement	0	0	1,230	0
Net periodic expense	(1,571)	(176)	(3,520)	(506)
Black Lung
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	598	751	1,762	1,558
Interest cost	747	1,567	2,493	3,386
Expected return on plan assets	(14)	(17)	(40)	(49)
Amortization of net actuarial loss (gain)	596	72	1,346	162
Net periodic expense	1,927	2,373	5,561	5,057
Life Insurance Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Interest cost	84	106	253	318
Amortization of net actuarial loss (gain)	(12)	(26)	(36)	(78)
Net periodic expense	$ 72	$ 80	$ 217	$ 240