Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (loss) - Reclassifications of Accumulated Other Comprehensive Loss (Details) - USD ($)		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Amortization of defined benefit pension and postretirement plan items:
Income tax benefit		$ (26,905,000)	$ (23,945,000)	$ (22,085,000)
Other Comprehensive Income Loss Adjustments AOCI Swap Agreements		(735,000)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Amortization of defined benefit pension and postretirement plan items:
Net of tax		(472,000)	(581,000)
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]	68,000	58,000
Net gain	[1]	(650,000)	(279,000)
Total before income taxes		(582,000)	(221,000)
Income tax benefit		233,000	89,000
Net of tax		(349,000)	(132,000)
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:
Total before income taxes		(206,000)	(752,000)
Income tax benefit		83,000	303,000
Net of tax		(123,000)	(449,000)
Other Comprehensive Income Loss Adjustments AOCI Swap Agreements	[2]	(120,000)	(735,000)
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]	(55,000)	(17,000)
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]	$ (31,000)	$ 0

[1] These amounts are included in the computation of net periodic benefits. See Note 16, Employee Benefit Plans, for additional details.

[2] These amounts are included in the effects of gains and losses from derivative instruments. See Note 7, Derivative Instruments, for additional details.