Debt and Financing Arrangements Long-Term Debt Instruments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 15, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-Term Debt	$ 3,194,679		$ 2,191,576
Long-term debt due within one year	251,541		$ 251,430
6.05% due 2018
Debt Instrument [Line Items]
Long-term debt due within one year	$ 250,000
Long-term debt, Interest Rate	6.05%
Dalton Expansion Project
Debt Instrument [Line Items]
Proceeds from Issuance of Other Long-term Debt	$ 18,800
Long-Term Debt	247,700
Long-term debt due within one year	$ 1,600
4%SeniorUnsecuredNotesDue2028
Debt Instrument [Line Items]
Long-term debt, Interest Rate		4.00%
Issuance of Long-Term Debt		$ 400,000
Additional Interest Rate Accrued For Default Of Registration Rights Agreements First Period		0.25%
Additional Interest Rate Accrued For Default Of Registration Rights Agreements Each Subsequent Period		0.25%
Maximum Additional Interest Rate Accrued For Default Of Registration Rights Agreements All Periods		0.50%
4.6%SeniorUnsecuredNotesDue2048
Debt Instrument [Line Items]
Long-term debt, Interest Rate		4.60%
Issuance of Long-Term Debt		$ 600,000
Additional Interest Rate Accrued For Default Of Registration Rights Agreements First Period		0.25%
Additional Interest Rate Accrued For Default Of Registration Rights Agreements Each Subsequent Period		0.25%
Maximum Additional Interest Rate Accrued For Default Of Registration Rights Agreements All Periods		0.50%