Fair Value of Financial Assets and Liabilities (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Fair Value of Financial Assets and Liabilities
Risk premium (as a percent)	1.19%
Advances for construction	$ 182,042	$ 183,393
Level 2
Fair Value of Financial Assets and Liabilities
Long-term debt, including current maturities	526,974	511,146
Advances for construction	74,749	73,389
Total liabilities	601,723	584,535
Total
Fair Value of Financial Assets and Liabilities
Long-term debt, including current maturities	526,974	511,146
Advances for construction	74,749	73,389
Total liabilities	601,723	584,535
Cost
Fair Value of Financial Assets and Liabilities
Long-term debt, including current maturities	433,590	434,050
Advances for construction	182,042	183,393
Total liabilities	$ 615,632	$ 617,443