Fair Value Disclosures (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	$ 42,672	$ 509,167	$ 281,158	$ 46,299
Long-Term Debt	2,184,481	2,187,289
Long-term Debt, Fair Value	2,169,446	2,281,282
Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	42,672	509,167
Long-Term Debt	$ 2,194,003	$ 2,204,825