Derivative Instruments Gain (Loss) by Hedging Relationship, by Income Statement Location, by Derivative Instrument Risk (Details)	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Nov. 01, 2015
Derivative [Line Items]
Target Hedge 2015-2016 Gas Year			75.00%
Unrealized Gain/Loss [Abstract]
Derivative, Loss on Derivative	$ 7,800,000	$ 26,900,000
Unrealized loss On Derivatives	22,300,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	2,900,000	18,800,000
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	$ 23,300,000	700,000
Maturity Of Credit Risk Derivatives	Mar. 31, 2020
Derivative Fair Value Of Derivative Net	$ 20,300,000	$ (18,100,000)
Notional Disclosures [Abstract]
Financial Derivative, Nonmonetary Notional Amount	429,100,000	477,430,000
Derivative, Nonmonetary Notional Amount	520,268,000	535,450,000
Notional Amount of Foreign Currency Derivatives	$ 7,669,000	$ 7,497,000
Foreign Exchange Contract [Member]
Derivative [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	0	0
Foreign Exchange Contract [Member] | Cost of Sales [Member]
Derivative [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	107,000	(130,000)
Foreign Exchange Contract [Member] | Sales [Member]
Derivative [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	0	0
Natural Gas Commodity [Member]
Derivative [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	(356,000)	347,000
Natural Gas Commodity [Member] | Cost of Sales [Member]
Derivative [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	(26,000,000)	(22,746,000)
Natural Gas Commodity [Member] | Sales [Member]
Derivative [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	(1,021,000)	995,000
Deferred Derivative Gain (Loss) [Member] | Foreign Exchange Contract [Member]
Derivative [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	(107,000)	130,000
Deferred Derivative Gain (Loss) [Member] | Natural Gas Commodity [Member]
Derivative [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	$ 26,665,000	$ (23,394,000)