Interest Expense (Details) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
	Mar. 31, 2017	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Interest expense, Net
Senior Notes			$ 27,621	$ 37,481	$ 38,250
Credit facility unutilized fee			3,795	4,132	2,753
Federal coal lease obligations imputed interest			350	342	2,960
Amortization of deferred financing costs and original issue discount			7,781	5,173	4,174
Other			233	257	442
Subtotal			39,780	47,385	48,579
Premium on early retirement of debt		$ (900)	880	0	0
Write-off of deferred financing costs and original issue discount		$ 700	702	1,254	0
Other			0	0	0
Subtotal - cost of early retirement of debt and refinancings			1,582	1,254	0
Total Interest Expense			$ 41,362	$ 48,639	$ 48,579
Senior 8.50 Percent Notes Due 2019 [Member]
Interest expense, Net
Write-off of deferred financing costs and original issue discount	$ 700
Equity Offering [Line Items]
Interest Paid	$ 1,500