Income Taxes (Deferred Tax Assets and Liabilities) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Deferred tax assets:
Benefit of tax loss carryforwards	$ 17,709	$ 7,798
Alternative minimum tax credit carryforward	21,638	21,599
Pensions and benefits	54,652	86,816
Asset retirement obligation	23,727	21,710
Deferred fuel	0	1,951
Other	14,485	14,115
Total gross deferred tax assets	132,211	153,989
Deferred tax liabilities:
Plant, principally due to depreciation and basis differences	(511,847)	(457,127)
Decommissioning	(35,489)	(29,416)
Deferred fuel	(2,171)	0
Other	(5,664)	(5,828)
Total gross deferred tax liabilities	(555,171)	(492,371)
Net accumulated deferred income taxes	$ (422,960)	$ (338,382)