Commitments and Contingencies Commitments and Contingencies (Details) (USD $)	12 Months Ended
	Dec. 31, 2013 MW	Dec. 31, 2012	Dec. 31, 2011
Long-term Purchase Commitment [Line Items]
Long-term Contract for Purchase of Electric Power, Date of Contract Expiration	Oct. 16, 2037
Contractual Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due within One Year	$ 203,131,000
Unrecorded Unconditional Purchase Obligation, Due within Two Years	148,874,000
Unrecorded Unconditional Purchase Obligation, Due within Three Years	136,503,000
Unrecorded Unconditional Purchase Obligation, Due within Four Years	125,492,000
Unrecorded Unconditional Purchase Obligation, Due within Five Years	110,930,000
Unrecorded Unconditional Purchase Obligation, Due after Five Years	148,362,000
Purchase Commitment [Member]
Long-term Purchase Commitment [Line Items]
Expiration Date of Contracts Maximum, Gas Supply	Dec. 31, 2017
Equipment [Member]
Long-term Purchase Commitment [Line Items]
Long Term Purchase Commitment Percentage Committed	100.00%
Construction Contracts [Member]
Long-term Purchase Commitment [Line Items]
Long Term Purchase Commitment Percentage Committed	75.00%
Cheyenne Prairie [Member]
Long-term Purchase Commitment [Line Items]
Long-term Purchase Commitment, Amount	222,000,000
PacifiCorp Purchase Power Agreement [Member]
Long-term Purchase Commitment [Line Items]
Number of M W Capacity Purchased Under Long-term Contract	50
Cost of Purchased Power	13,026,000	13,224,000	12,515,000
Long-term Contract for Purchase of Electric Power, Date of Contract Expiration	Dec. 31, 2023
Public Service of Colorado Purchase Power Agreement [Member]
Long-term Purchase Commitment [Line Items]
Cost of Purchased Power	0	0	97,988,000	[1]
PacifiCorp Transmission [Member]
Long-term Purchase Commitment [Line Items]
Number of M W Capacity Purchased Under Long-term Contract	50
Cost of Purchased Power	1,384,000	1,215,000	1,215,000
Long-term Contract for Purchase of Electric Power, Date of Contract Expiration	Dec. 31, 2023
Happy Jack Wind Purchase Power Agreeement [Member]
Long-term Purchase Commitment [Line Items]
Number of M W Capacity Purchased Under Long-term Contract	30
Long-term Contract for Purchase of Electric Power, Date of Contract Expiration	Sep. 03, 2028
Silver Sage Wind Power Purchase Agreement [Member]
Long-term Purchase Commitment [Line Items]
Number of M W Capacity Purchased Under Long-term Contract	30
Long-term Contract for Purchase of Electric Power, Date of Contract Expiration	Sep. 30, 2029
Busch Ranch Wind Farm [Member]
Long-term Purchase Commitment [Line Items]
Number of M W Capacity Purchased Under Long-term Contract	14.5
Cost of Purchased Power	1,856,000	502,000	0
Cargill Power Purchase Agreement [Member]
Long-term Purchase Commitment [Line Items]
Number of M W Capacity Purchased Under Long-term Contract	50
Long-term Contract for Purchase of Electric Power, Date of Contract Expiration	Dec. 31, 2014
Basin Electric [Member]
Long-term Purchase Commitment [Line Items]
Number of M W Capacity Purchased Under Long-term Contract	40
Long-term Contract for Purchase of Electric Power, Date of Contract Expiration	Sep. 30, 2014
Renewable Wind Energy, Cheyenne Light [Member] | Happy Jack Wind Purchase Power Agreeement [Member]
Long-term Purchase Commitment [Line Items]
Cost of Purchased Power	3,772,000	1,988,000	1,955,000
Renewable Wind Energy, Cheyenne Light [Member] | Silver Sage Wind Power Purchase Agreement [Member]
Long-term Purchase Commitment [Line Items]
Cost of Purchased Power	$ 4,809,000	$ 3,269,000	$ 3,281,000
Wygen I Generating Facility [Member] | Purchase Option, Property [Member]
Long-term Purchase Commitment [Line Items]
Number of M W Capacity Purchased Under Long-term Contract	60
Long-term Contract for Purchase of Electric Power, Date of Contract Expiration	Dec. 31, 2022

[1] This PPA with PSCo expired on Dec. 31, 2011 and was replaced with the facilities constructed by Colorado Electric and Black Hills Colorado IPP at our Pueblo Airport Generation site. The facilities constructed by Black Hills Colorado IPP were to support an inter-company PPA with Colorado Electric. This inter-company PPA is being accounted for as a capital lease.