Principles Of Preparation (Supplemental Cash Flow Disclosures) (Details) - USD ($) $ in Thousands			9 Months Ended
		Jul. 06, 2016	Sep. 30, 2017	Sep. 30, 2016
Supplemental Cash Flow Information [Abstract]
Interest on long-term debt and borrowings under the revolving credit facility			$ 47,412	$ 46,867
Income tax paid, net			1,576	3,337
Sale of interest in Four Corners Generating Station	[1]		0	27,720
Changes in accrued plant additions			(6,228)	4,277
Site Contingency [Line Items]
Proceeds from Sale of Property, Plant, and Equipment			0	3,251
Four Corners Generating Station
Site Contingency [Line Items]
Property, Plant and Equipment, Disposals		$ 32,000
Asset Retirement Obligation		7,000
Mine Reclamation and Closing Liability, Noncurrent		19,500
Public Utilities, Property, Plant and Equipment, Amount of Disposal Adjustments		1,300
Proceeds from Sale of Property, Plant, and Equipment		$ 4,200
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock issued			1,171	1,236
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock issued			$ 932	$ 0

[1] The Company sold its interest in Four Corners Generating Station ("Four Corners") in July 2016 for approximately $32.0 million based on book value as defined in the purchase and sale agreement related to the sale. The sales price was adjusted downward by $7.0 million and $19.5 million to reflect Arizona Public Service Company's ("APS") affiliate assumption of the Company's obligation to pay for future plant decommissioning and mine reclamation expense, respectively. The sales price was also adjusted downward by approximately $1.3 million for closing adjustments and other assets and liabilities assumed by APS's affiliate. At the closing of the sale on July 6, 2016, the Company received approximately $4.2 million in cash.