Income Taxes (Net Deferred Tax Liability) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Deferred tax liabilities:
Differences between book and tax basis of property	$ 1,441,149	$ 1,916,619
Cash flow hedges	112,625	276,063
Other	4,460	15,755
Total deferred tax liabilities	1,558,234	2,208,437
Deferred tax assets
Accrued compensation	16,387	30,316
Alternative minimum tax credit carryforward	89,016	73,516
Stored natural gas	7,812	9,053
Accrued pension costs	7,686	3,982
Asset retirement obligations	39,249	13,188
Net operating loss carryforward	217,276	287,830
Differences between book and tax basis of property - state	16,872
Other	12,354	9,591
Total deferred tax assets	406,652	427,476
Net deferred tax liability	$ 1,151,582	$ 1,780,961