Income Taxes (Net Deferred Tax Liability) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Deferred tax liabilities:
Differences between book and tax basis of property	$ 2,114,685	$ 1,441,149
Cash flow hedges	13,171	112,625
Other	10,715	4,460
Total deferred tax liabilities	2,138,571	1,558,234
Deferred tax assets
Accrued compensation	15,718	16,387
Alternative minimum tax credit carryforward	76,864	89,016
Stored natural gas	9,272	7,812
Accrued pension costs	1,821	7,686
Asset retirement obligations	53,419	39,249
Net operating loss carryforward	411,994	217,276
Differences between book and tax basis of property - state	11,045	16,872
Other	7,405	12,354
Total deferred tax assets	587,538	406,652
Net deferred tax liability	$ 1,551,033	$ 1,151,582