Fair Value of Financial Assets and Liabilities (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2014	Dec. 31, 2013
Fair Value of Financial Assets and Liabilities
Risk premium (as a percent)	1.19%
Advances for construction	$ 182,172	$ 183,393
Cost
Fair Value of Financial Assets and Liabilities
Long-term debt, including current maturities	429,444	434,050
Advances for construction	182,172	183,393
Total liabilities	611,616	617,443
Total
Fair Value of Financial Assets and Liabilities
Long-term debt, including current maturities	526,733	511,146
Advances for construction	74,389	73,389
Total liabilities	601,122	584,535
Level 2
Fair Value of Financial Assets and Liabilities
Long-term debt, including current maturities	526,733	511,146
Advances for construction	74,389	73,389
Total liabilities	$ 601,122	$ 584,535