Employee Benefit Plans - Schedule of Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Interest cost	$ 4,735	$ 6,616
Expected return on plan assets	(6,812)	(7,006)
Amortization of net actuarial loss (gain)	469	208
Settlement	1,167	0
Net periodic expense	(441)	(182)
Black Lung
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	579	404
Interest cost	898	910
Expected return on plan assets	(13)	(16)
Amortization of net actuarial loss (gain)	331	45
Net periodic expense	1,795	1,343
Life Insurance Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Interest cost	84	106
Amortization of net actuarial loss (gain)	(12)	(26)
Net periodic expense	$ 72	$ 80