Supplemental Statements of Cash Flows Disclosures Supplemental Statements of Cash Flows Disclosures (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016		Dec. 31, 2015
Supplemental Cash Flow Information [Abstract]
Interest on long-term debt and borrowing under the revolving credit facility	$ 70,523	$ 69,990		$ 62,297
Income taxes paid, net	2,055	2,328		1,000
Sale of Interest in Four Corners Generating Station	0	27,720	[1]	0
Changes in accrued plant additions	5,090	(4,789)		(6,660)
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock based compensation, grants of common stock	1,171	1,235		1,567
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock based compensation, grants of common stock	$ 932	$ 0		$ 0

[1] The Company sold its interest in Four Corners in July 2016. The sales proceeds were reduced by the settlement of other obligations between the Company and APS and its affiliate, 4C Acquisition, LLC. See Part II, Item 8, Financial Statements and Supplementary Data, Note E.