Significant Accounting Policies, Interest Rate Swap Agreements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Interest Rate Swap Agreement [Abstract]
Interest rate swap settlements reclassified from regulatory assets to interest expense	$ 301	$ 345	$ 366
Interest rate swap gain (loss) capitalized	(209)	$ (128)	$ (285)
Interest rate swap settlements to be reclassified during the next 12 months	$ 252
Interest rate swap expiration date	Oct. 01, 2029