Income Taxes (Net Deferred Tax Liability) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Deferred tax liabilities:
Differences between book and tax basis of property	$ 1,916,619	$ 1,411,240
Cash flow hedges	276,063	61,394
Other	15,755	14,122
Total deferred tax liabilities	2,208,437	1,486,756
Deferred tax assets:
Accrued compensation	30,316	16,279
Alternative minimum tax credit carryforward	73,516	70,138
Stored natural gas	9,053	7,145
Accrued pension costs	3,982	6,227
Asset retirement obligations	13,188	10,848
Net operating loss carryforward	287,830	192,086
Other	9,591	9,652
Total deferred tax assets	427,476	312,375
Net deferred tax liability	1,780,961	1,174,381
Canada [Member]
Deferred tax assets:
Net operating loss carryforward	$ 11,000