Basis of Financial Reporting Revenue Disclosures (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Regulatory Liability [Domain]
Cumulative Effect of New Accounting Principle in Period of Adoption	$ 12,100
Regulatory Liability [Domain] | Accounting Standards Update 2014-09 [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	12,107
Accounts Payable [Member] | Accounting Standards Update 2014-09 [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	8,747
Accrued Liabilities [Member] | Accounting Standards Update 2014-09 [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	22,794
Deferred Income Tax Charge [Member] | Accounting Standards Update 2014-09 [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	$ 44,575