Operations and Significant Accounting Policies - Investments and Other (Details)	12 Months Ended
Dec. 31, 2012
Long-term Contract for Purchase of Electric Power [Line Items]
Real Estate Sales Revenue Recognition, Minimum Contract Price Cash Collections Requirement	20.00%
Square Butte PPA [Member]
Long-term Contract for Purchase of Electric Power [Line Items]
Output Purchased (Percent)	50.00%
Output Purchased (MW)	227.5

Share of plant output being purchased under a long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 980

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SAB TOPIC 10.D)

-URI http://asc.fasb.org/extlink&oid=6499423&loc=d3e659339-123030

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 10

-Section D

Name:	us-gaap_LongTermContractForPurchaseOfElectricPowerShareOfPlantOutputBeingPurchased
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration