Employee Benefit Plans - Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive (Income) Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actuarial loss (gain)	$ 42,891	$ 22,895	$ 3,832
Amortization of net actuarial (loss) gain	(7,405)	(155)	203
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actuarial loss (gain)	30,514	23,075
Amortization of net actuarial (loss) gain	(797)	0
Settlement	(6,224)	0
Total recognized in other comprehensive loss	23,493	23,075
Black lung
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actuarial loss (gain)	11,512	255	3,661
Amortization of net actuarial (loss) gain	(216)	(199)	149
Total recognized in other comprehensive loss	11,296	56	$ 3,810
Life insurance benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actuarial loss (gain)	1,002	(1,164)
Amortization of net actuarial (loss) gain	105	46
Total recognized in other comprehensive loss	1,107	$ (1,118)
Alpha Companies | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actuarial loss (gain)	1,686
Alpha Companies | Black lung
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actuarial loss (gain)	$ (344)