Reclassification out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended		12 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, for Net Gain (Loss), Net of Tax		$ (487)	$ (384)	$ (1,475)	$ (1,723)
Other Comprehensive Income (Loss), Finalization of Pension and Other Postretirement Benefit Plan Valuation, Net of Tax			800
Period opening balance				(51,753)
Net current period other comprehensive income ending balance		257	1,759	784	2,585
Period closing balance		(50,969)		(50,969)		$ (51,753)
Other Comprehensive (Income) Loss, Amortization Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, for Net Prior Service Cost (Credit), Net of Tax		(230)	(257)	(691)	(770)
Other comprehensive income (loss), tax				500	600
Defined Benefit Plan, Settlements, Benefit Obligation					3,500
Other Comprehensive Income (Loss), Finalization of Pension and Other Postretirement Benefit Plan Valuation, Tax		0	1,054	0	(1,054)
Defined Benefit Pension Plan Adjustments
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax						953
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax						1,632
Period opening balance			(52,963)	(51,753)	(55,548)	(55,548)
Net current period other comprehensive income ending balance	[1]			784	2,585
Period closing balance		(50,969)		(50,969)		$ (51,753)
Pension Plan [Member]
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Defined Benefit Plan, Settlements, Benefit Obligation		$ 0	$ 3,482	$ 0	$ 3,482

[1] Included in other comprehensive income are net periodic costs associated with our pension and other postretirement employee benefit (OPEB) plans that were reclassified from accumulated other comprehensive loss. For the nine months ended September 30, 2017 and 2016, actuarial loss amortization of $1.5 million and $1.7 million, respectively, as well as $0.7 million and $0.8 million, respectively, of prior service credit amortization were reclassified. These amounts are net of tax totaling $0.5 million and $0.6 million for each respective period. These accumulated other comprehensive loss components are included in the computation of net periodic pension and OPEB costs in Note 10, “Pension and Other Postretirement Employee Benefit Plans.” 2 Included in "Amounts reclassified from accumulated other comprehensive loss" above for the nine months ended September 30, 2016 is settlement expense of $3.5 million associated with the remeasurement of our salaried pension plan, which is discussed further in Note 10, “Pension and Other Postretirement Employee Benefit Plans.” The remeasurement resulted in a settlement loss of $0.8 million recorded to the pension liability and reclassified from accumulated other comprehensive loss. The settlement expense and corresponding remeasurement are net of tax totaling $1.1 million.