Rate Matters (Details) (USD $)	1 Months Ended	3 Months Ended	12 Months Ended	1 Months Ended	0 Months Ended	1 Months Ended
	Oct. 31, 2014	Mar. 31, 2015	Dec. 31, 2014	Dec. 31, 2014	Mar. 03, 2015	Feb. 28, 2015	Apr. 30, 2015
Demand Side Management Cost Adjustment, 2015 [Member]
Rate Matters [Abstract]
Public Utilities, Proposed electric demand side management budget	$ 81,600,000
Public Utilities, Proposed gas demand side management budget	13,100,000
Demand Side Management Cost Adjustment [Member]
Rate Matters [Abstract]
Public Utilities, Incentive award upon achieving savings goal		5,000,000
Public Utilities, Percentage of net economic benefits on which incentive is earned		5.00%
Public Utilities, Maximum annual incentive		30,000,000
Demand Side Management Cost Adjustment, 2016 [Member]
Rate Matters [Abstract]
Public Utilities, Proposed electric demand side management budget	78,700,000
Public Utilities, Proposed gas demand side management budget	13,600,000
CPUC Proceeding - Colorado 2014 Electric Rate Case
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Amount			136,000,000
Public Utilities, Requested Rate Increase (Decrease), Percentage			4.83%
CPUC Proceeding - Colorado 2014 Electric Rate Case, Electric Rates 2015
Rate Matters [Abstract]
Public Utilities, Portion of requested rate increase (decrease) attributable to CACJA recovery			100,900,000
Public Utilities, Portion of requested rate increase (decrease) attributable to CACJA recovery, Amended			98,700,000
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount				107,200,000
Public Utilities, Requested Rate Increase (Decrease), Amended, Percentage				3.79%
Public Utilities, Requested Return on Equity, Revised, Percentage				10.25%
CPUC Proceeding - Colorado 2014 Electric Rate Case, Rider Revenue 2016
Rate Matters [Abstract]
Public Utilities, Requested increase (decrease) to rider revenue			34,200,000
CPUC Proceeding - Colorado 2014 Electric Rate Case, Rider Revenue 2017
Rate Matters [Abstract]
Public Utilities, Requested increase (decrease) to rider revenue			29,900,000
CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case, Gas Rates 2015 [Member]
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Amount					40,500,000
Public Utilities, Requested Rate Increase (Decrease), Percentage					3.50%
Public Utilities, Requested Return on Equity, Percentage					10.10%
Public Utilities, Requested Rate Base, Amount					1,260,000,000
Public Utilities, Revenue impact of requested PSIA rider					(100,000)
Public Utilities, Revenue projection of requested PSIA rider					67,000,000
Public Utilities, Plant and plant related expenses					24,400,000
Public Utilities, Operating and maintenance expenses					23,900,000
Public Utilities, Property and payroll taxes					4,700,000
Public Utilities, ROE					4,500,000
Public Utilities, Capital structure					(1,000,000)
Public Utilities, Sales forecast					(17,100,000)
Public Utilities, Other net					1,100,000
Public Utilities, Total base rate increase					40,500,000
Public Utilities, Incremental PSIA rider revenues					(100,000)
Public Utilities, Total revenue impact					40,400,000
CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case, Gas Rates 2016 [Member]
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Amount					7,600,000
Public Utilities, Requested Rate Increase (Decrease), Percentage					0.70%
Public Utilities, Requested Rate Base, Amount					1,310,000,000
Public Utilities, Revenue impact of requested PSIA rider					21,700,000
Public Utilities, Revenue projection of requested PSIA rider					88,700,000
Public Utilities, Plant and plant related expenses					12,400,000
Public Utilities, Operating and maintenance expenses					(5,200,000)
Public Utilities, Property and payroll taxes					2,600,000
Public Utilities, ROE					0
Public Utilities, Capital structure					0
Public Utilities, Sales forecast					(2,200,000)
Public Utilities, Other net					0
Public Utilities, Total base rate increase					7,600,000
Public Utilities, Incremental PSIA rider revenues					21,700,000
Public Utilities, Total revenue impact					29,300,000
CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case, Gas Rates 2017 [Member]
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Amount					18,100,000
Public Utilities, Requested Rate Increase (Decrease), Percentage					1.50%
Public Utilities, Requested Return on Equity, Percentage					10.30%
Public Utilities, Requested Rate Base, Amount					1,360,000,000
Public Utilities, Revenue impact of requested PSIA rider					21,200,000
Public Utilities, Revenue projection of requested PSIA rider					109,900,000
Public Utilities, Plant and plant related expenses					12,000,000
Public Utilities, Operating and maintenance expenses					600,000
Public Utilities, Property and payroll taxes					4,000,000
Public Utilities, ROE					2,400,000
Public Utilities, Capital structure					100,000
Public Utilities, Sales forecast					(1,000,000)
Public Utilities, Other net					0
Public Utilities, Total base rate increase					18,100,000
Public Utilities, Incremental PSIA rider revenues					21,200,000
Public Utilities, Total revenue impact					39,300,000
CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case [Member]
Rate Matters [Abstract]
Public Utilities, Requested Equity Capital Structure, Percentage					56.00%
CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case [Member] | Shareholders
Public Utilities, General Disclosures [Line Items]
Public Utilities, Percentage share in earnings for ROE recognized within earnings thresholds					50.00%
CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case [Member] | Customers
Public Utilities, General Disclosures [Line Items]
Public Utilities, Percentage share in earnings for ROE recognized within earnings thresholds					50.00%
CPUC Proceeding - Annual Electric Earnings Test
Rate Matters [Abstract]
Public Utilities, Return on equity threshold triggering earnings sharing, Percentage		10.00%				9.83%
Colorado Public Utilities Commission (CPUC) | Demand Side Management Cost Adjustment 2014 [Member]
Rate Matters [Abstract]
Public Utilities, Maximum savings goal (in GWh)			384
Colorado Public Utilities Commission (CPUC) | Demand Side Management Cost Adjustment, 2015 [Member]
Rate Matters [Abstract]
Public Utilities, Maximum savings goal (in GWh)		400
Colorado Public Utilities Commission (CPUC) | CPUC Proceeding - Colorado 2014 Electric Rate Case, Electric Rates 2015
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Equity Capital Structure, Percentage						56.00%
Public Utilities, Revenue impact of CACJA rider mechanism						97,000,000
Public Utilities, Revenue impact of transmission cost adjustment rider						15,600,000
Public Utilities, Rider costs not to be recovered through base rates, amount						(19,900,000)
Rate Matters [Abstract]
Public Utilities, Approved Rate Increase (Decrease), Amount						53,300,000
Public Utilities, Base rate increase (decrease)						(39,400,000)
Public Utilities, Approved Rate Increase (Decrease), Percentage						1.87%
Public Utilities, Settlement Agreement, Requested Return on Equity, Percentage						9.83%
Public Utilities, Deferral of additional expenses due to approved rate case						3,100,000
Colorado Public Utilities Commission (CPUC) | CPUC Proceeding - Colorado 2014 Electric Rate Case, Electric Rates 2015 | Shareholders
Public Utilities, General Disclosures [Line Items]
Public Utilities, Percentage share in earnings for ROE recognized within earnings thresholds						50.00%
Colorado Public Utilities Commission (CPUC) | CPUC Proceeding - Colorado 2014 Electric Rate Case, Electric Rates 2015 | Customers
Public Utilities, General Disclosures [Line Items]
Public Utilities, Percentage share in earnings for ROE recognized within earnings thresholds						50.00%
Colorado Public Utilities Commission (CPUC) | CPUC Proceeding - Colorado 2014 Electric Rate Case, Rider Revenue 2016
Public Utilities, General Disclosures [Line Items]
Public Utilities, Revenue impact of CACJA rider mechanism						17,700,000
Colorado Public Utilities Commission (CPUC) | CPUC Proceeding - Colorado 2014 Electric Rate Case, Rider Revenue 2017
Public Utilities, General Disclosures [Line Items]
Public Utilities, Revenue impact of CACJA rider mechanism						14,500,000
Subsequent Event | Colorado Public Utilities Commission (CPUC) | CPUC Proceeding - 2013 Electric Earnings Test
Rate Matters [Abstract]
Public Utilities, Refund to customers due to annual earnings test							$ 66,500,000
Minimum | CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case, Gas Rates 2016 [Member]
Rate Matters [Abstract]
Public Utilities, Return on equity threshold triggering earnings sharing, Percentage					10.20%
Minimum | CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case, Gas Rates 2017 [Member]
Rate Matters [Abstract]
Public Utilities, Return on equity threshold triggering earnings sharing, Percentage					10.40%
Minimum | Colorado Public Utilities Commission (CPUC) | CPUC Proceeding - Colorado 2014 Electric Rate Case, Electric Rates 2015
Rate Matters [Abstract]
Public Utilities, Return on equity threshold triggering earnings sharing, Percentage						9.84%
Maximum | CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case, Gas Rates 2016 [Member]
Rate Matters [Abstract]
Public Utilities, Return on equity threshold triggering earnings sharing, Percentage					10.60%
Maximum | CPUC Proceeding - Colorado 2015 Multi-Year Gas Rate Case, Gas Rates 2017 [Member]
Rate Matters [Abstract]
Public Utilities, Return on equity threshold triggering earnings sharing, Percentage					10.80%
Maximum | Colorado Public Utilities Commission (CPUC) | CPUC Proceeding - Colorado 2014 Electric Rate Case, Electric Rates 2015
Rate Matters [Abstract]
Public Utilities, Return on equity threshold triggering earnings sharing, Percentage						10.48%