Fair Value of Financial Assets and Liabilities (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2014	Dec. 31, 2013
Fair Value of Financial Assets and Liabilities
Risk premium (as a percent)	1.19%
Advances for construction	$ 181,443	$ 183,393
Level 2
Fair Value of Financial Assets and Liabilities
Long-term debt, including current maturities	526,155	511,146
Advances for construction	74,365	73,389
Total liabilities	600,520	584,535
Total
Fair Value of Financial Assets and Liabilities
Long-term debt, including current maturities	526,155	511,146
Advances for construction	74,365	73,389
Total liabilities	600,520	584,535
Cost
Fair Value of Financial Assets and Liabilities
Long-term debt, including current maturities	429,884	434,050
Advances for construction	181,443	183,393
Total liabilities	$ 611,327	$ 617,443