Accumulated Other Comprehensive Income (Loss) (Details 1) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of prior service costs (credits), before tax	$ 612	$ 1,821	$ 4,287	$ 5,462
Postretirement medical plan termination	19,477	0	19,477	0
Postretirement medical plan | Amount Reclassified from Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of prior service costs (credits), before tax	(612)	(1,821)	(4,287)	(5,462)
Postretirement medical plan termination	(19,477)	0	(19,477)	0
Total before tax	(20,089)	(1,821)	(23,764)	(5,462)
Tax expense (benefit)	4,620	0	5,466	0
Amounts reclassified from AOCI	$ (15,469)	$ (1,821)	$ (18,298)	$ (5,462)