Stockholders' Equity Accumulated Other Comprehensive Income (loss) - Reclassifications of Accumulated Other Comprehensive Loss (Details) - USD ($)		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Amortization of defined benefit pension and postretirement plan items:
Other Comprehensive Income Loss Adjustments AOCI Swap Agreements			$ (735,000)
Income tax benefit		$ (28,341,000)	(26,905,000)	$ (23,945,000)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Amortization of defined benefit pension and postretirement plan items:
Net of tax		(454,000)	(472,000)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Defined Benefit Plans Adjustment [Member]
Amortization of defined benefit pension and postretirement plan items:
Prior service cost	[1]	77,000	68,000
Net gain	[1]	(871,000)	(650,000)
Total before income taxes		(794,000)	(582,000)
Income tax benefit		320,000	233,000
Net of tax		(474,000)	(349,000)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated (Gain) Loss from Commodity Contracts Cash Flows Hedges [Member]
Amortization of defined benefit pension and postretirement plan items:
Other Comprehensive Income Loss Adjustments AOCI Swap Agreements	[2]	(322,000)	(120,000)
Total before income taxes		23,000	(206,000)
Income tax benefit		(3,000)	83,000
Net of tax		20,000	(123,000)
Call Options [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated (Gain) Loss from Commodity Contracts Cash Flows Hedges [Member]
Amortization of defined benefit pension and postretirement plan items:
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax	[2]	0	(55,000)
Natural Gas Futures [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated (Gain) Loss from Commodity Contracts Cash Flows Hedges [Member]
Amortization of defined benefit pension and postretirement plan items:
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax	[2]	$ 345,000	$ (31,000)

[1] Includes 76,745, 70,631 and 57,382 shares at December 31, 2016, 2015 and 2014, respectively, held in a Rabbi Trust related to our Non-Qualified Deferred Compensation Plan.

[2] Includes amounts for shares issued for directors’ compensation.