Significant Accounting Policies Retirement Benefits ASU 2017-07 (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operations and maintenance	$ 38,028	$ 34,997	$ 77,551	$ 72,443
Accounting Standards Updates 2017-07 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operations and maintenance	$ 1,300		$ 2,600

The amount of operating expenses for the period that are associated with the utility's normal revenue producing operation; including selling, general, and administrative expenses, costs incurred for routine plant repairs and maintenance, and operations.

No definition available.

Name:	us-gaap_UtilitiesOperatingExpenseMaintenanceOperationsAndOtherCostsAndExpenses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration