Income Taxes (Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets:
Benefit of tax loss carryforwards	$ 60,749	$ 35,153
Alternative minimum tax credit carryforward	16,620	16,620
Pensions and benefits	57,756	61,673
Asset retirement obligation	26,929	28,042
Deferred fuel	0	1,488
Other	(200)	15,421
Total gross deferred tax assets	161,854	158,397
Deferred tax liabilities:
Plant, principally due to depreciation and basis differences	(668,303)	(608,738)
Decommissioning	(43,463)	(41,100)
Deferred fuel	3,962	0
Other	(1,192)	(3,796)
Total gross deferred tax liabilities	(716,920)	(653,634)
Net accumulated deferred income taxes	$ (555,066)	$ (495,237)