Income Taxes (Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Benefit of tax loss carryforwards	$ 0	$ 12,521
Alternative minimum tax credit carryforward	0	8,855
Pensions and benefits	33,456	31,874
Asset retirement obligation	23,239	21,305
Other	0	2,673
Total gross deferred tax assets	123,922	143,089
Deferred tax liabilities:
Plant, principally due to depreciation and basis differences	(432,073)	(437,465)
Decommissioning	(39,737)	(30,757)
Other	(510)	0
Total gross deferred tax liabilities	(472,320)	(468,222)
Net accumulated deferred income taxes	(348,398)	(325,133)
Regulatory Liabilities related to income taxes [Domain]
Income Tax [Line Items]
Deferred Tax Assets, Regulatory Assets	63,103	63,378
Deferred fuel [Domain]
Income Tax [Line Items]
Deferred Tax Assets, Regulatory Assets	$ 4,124	$ 2,483