Borrowings and Other Financing Instruments Borrowings and Other Financing Instruments, Long-Term Borrowings and Other Financing Instruments (Details) (USD $)	1 Months Ended	12 Months Ended
	Jun. 30, 2014	Dec. 31, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Deferred Finance Costs, Noncurrent, Net		4,300,000	$ 3,500,000
Maximum annual dividends that can be paid if equity capitalization ratio condition is not met		33,300,000
Minimum calendar year average equity to total capitalization ratio authorized by state commission		52.50%
Calendar year average equity to total capitalization ratio		52.80%
Unrestricted Retained Earnings Per State Regulatory Commissions Dividend Restrictions		8,300,000
First Mortgage Bonds | Series Due June 15, 2024
Debt Instrument [Line Items]
Face Amount	100,000,000
Interest Rate, Stated Percentage	3.30%	3.30%
Maturity Date	Jun. 15, 2024	Jun. 15, 2024
First Mortgage Bonds | Series Due Oct. 1, 2018 [Member]
Debt Instrument [Line Items]
Interest Rate, Stated Percentage		5.25%
Maturity Date		Oct. 01, 2018
Long-term Debt, Maturities, Repayments of Principal in Year Four		150,000,000