Income Taxes (Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Benefit of tax loss carryforwards	$ 24,035	$ 60,749
Alternative minimum tax credit carryforward	16,620	16,620
Pensions and benefits	32,606	57,756
Asset retirement obligation	19,530	26,929
Total gross deferred tax assets	157,990	162,054
Deferred tax liabilities:
Plant, principally due to depreciation and basis differences	(426,077)	(668,303)
Decommissioning	(34,520)	(43,463)
Deferred fuel	0	3,962
Other	(2,416)	(1,392)
Total gross deferred tax liabilities	(463,013)	(717,120)
Net accumulated deferred income taxes	(305,023)	(555,066)
Regulatory Liabilities related to income taxes [Domain]
Income Tax [Line Items]
Deferred Tax Assets, Regulatory Assets	63,794	0
Deferred fuel [Domain]
Income Tax [Line Items]
Deferred Tax Assets, Regulatory Assets	$ 1,405	$ 0