Segment Reporting Assets (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012		Dec. 31, 2011		Jun. 30, 2011
Segment Reporting, Asset Reconciling Item
Total Assets	$ 3,774,280		$ 4,127,083		$ 3,817,160
Electric Utilities [Member]
Segment Reporting, Asset Reconciling Item
Total Assets	2,300,948	[1]	2,254,914	[1]	1,900,806
Gas Utilities [Member]
Segment Reporting, Asset Reconciling Item
Total Assets	684,545		746,444		659,349
Oil and Gas [Member]
Segment Reporting, Asset Reconciling Item
Total Assets	416,617		425,970		366,270
Power Generation [Member]
Segment Reporting, Asset Reconciling Item
Total Assets	122,856	[1]	129,121	[1]	353,794
Coal Mining [Member]
Segment Reporting, Asset Reconciling Item
Total Assets	90,021		88,704		89,627
Unallocated Amount to Segment [Member]
Segment Reporting, Asset Reconciling Item
Total Assets	159,293		141,079	[2]	88,645	[2]
Discontinued Operation or Asset Disposal [Member]
Segment Reporting, Asset Reconciling Item
Total Assets	$ 0		$ 340,851	[3]	$ 358,669	[3]

[1] The PPA under which the new generating facility was constructed at our Pueblo Airport Generation site by Colorado IPP to support Colorado Electric customers is accounted for as a capital lease. Therefore, commencing December 31, 2011, assets previously recorded at Power Generation are now accounted for at Colorado Electric as a capital lease.

[2] Assets of the Corporate segment were restated due to deferred taxes that were not classified as discontinued operations.

[3] See Note 18 for further information relating to discontinued operations.