Principles Of Preparation (Supplemental Cash Flow Disclosures) (Details) - USD ($) $ in Thousands			9 Months Ended
		Jul. 06, 2016	Sep. 30, 2016	Sep. 30, 2015
Supplemental Cash Flow Information [Abstract]
Interest on long-term debt and borrowing under the revolving credit facility			$ 46,867	$ 41,406
Income tax paid (refunded), net			3,337	(272)
Sale of Interest in Four Corners Generating Station	[1]		27,720	0
Changes in accrued plant additions			4,277	(13,150)
Site Contingency [Line Items]
Proceeds from Sale of Property, Plant, and Equipment			3,251	644
Four Corners Generating Station
Site Contingency [Line Items]
Property, Plant and Equipment, Disposals		$ 32,000
Asset Retirement Obligation		7,000
Mine Reclamation 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
Accounts Payable		$ 1,600
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grants of restricted shares of common stock			$ 1,236	$ 1,106

[1] The Company sold its interest in the Four Corners Generating Station ("Four Corners") for approximately $32.0 million based on the book value as defined in the asset purchase agreement entered into by the Company and Arizona Public Service Company ("APS") on February 17, 2015 (the "Purchase and Sale Agreement"). The sales price was adjusted downward by $7.0 million and $19.5 million, respectively, to reflect APS assumption of the Company's obligation to pay for future plant decommissioning and mine reclamation expense. The sales price was also adjusted downward by approximately $1.3 million for closing adjustments and other assets and liabilities assumed by APS. At the closing of the sale, the Company received approximately $4.2 million in cash, subject to post-closing adjustments. Subsequently, the Company recorded post-closing adjustments to reflect adjustments to estimated capital expenditures and other assets and liabilities assumed by APS through July 6, 2016, which resulted in a $1.6 million refund due to APS.