Employee Benefit Plans Unrecognized loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance	$ (38,449)	$ (22,017)	$ (17,722)
Unrecognized gains arising in current year	(609)	(931)	4,680
Ending Balance	(32,974)	(38,449)	(22,017)
Unrecognized Pension and Postretirement Plan Income (Cost)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance	(15,675)	(15,961)	(22,505)
Ending Balance	(14,923)	(15,675)	(15,961)
Pension Plans | Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member]
Before tax
AOCI before Tax, Attributable to Parent	30,169	30,396
Reclass adjustment for postretirement plan gain included in net income	(2,652)	(2,420)
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, before Tax	1,042	2,193
AOCI before Tax, Attributable to Parent	28,559	30,169	30,396
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance	19,610	19,758
Recognized component of periodic pension cost	(1,724)	(1,573)
Unrecognized gains arising in current year	678	1,425
Ending Balance	18,564	19,610	19,758
Other Postretirement Benefit Plans | Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost [Member]
Before tax
AOCI before Tax, Attributable to Parent	(4,869)	(5,334)
Reclass adjustment for postretirement plan gain included in net income	465	465
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, before Tax	0
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, Prior to Curtailment, before Tax		0
AOCI before Tax, Attributable to Parent	(4,404)	(4,869)	(5,334)
Other Postretirement Benefit Plans | Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member]
Before tax
AOCI before Tax, Attributable to Parent	(1,183)	(508)
Reclass adjustment for postretirement plan gain included in net income	101	86
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, before Tax	(77)
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, Prior to Curtailment, before Tax		(761)
AOCI before Tax, Attributable to Parent	(1,159)	(1,183)	(508)
Other Postretirement Benefit Plans | Unrecognized Pension and Postretirement Plan Income (Cost)
Before tax
AOCI before Tax, Attributable to Parent	(6,052)	(5,842)
Reclass adjustment for postretirement plan gain included in net income	566	551
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, before Tax	(77)
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, Prior to Curtailment, before Tax		(761)
AOCI before Tax, Attributable to Parent	(5,563)	(6,052)	(5,842)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance	(3,935)	(3,798)
Recognized component of periodic pension cost	368
Unrecognized gains arising in current year	(50)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, Prior to Curtailment, Net of Tax		358
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, Prior to Curtailment, Net of Tax		(495)
Ending Balance	$ (3,617)	$ (3,935)	$ (3,798)