Fair Value of Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Fair Value of Financial Assets and Liabilities
Risk premium (as a percent)	1.19%
Advances for construction	$ 180,805	$ 182,284
Level 1
Fair Value of Financial Assets and Liabilities
Long-term debt, including current maturities	0	0
Advances for construction	0	0
Total	0	0
Level 2
Fair Value of Financial Assets and Liabilities
Long-term debt, including current maturities	518,301	534,068
Advances for construction	73,305	72,571
Total	591,606	606,639
Level 3
Fair Value of Financial Assets and Liabilities
Long-term debt, including current maturities	0	0
Advances for construction	0	0
Total	0	0
Cost
Fair Value of Financial Assets and Liabilities
Long-term debt, including current maturities	423,012	425,840
Advances for construction	180,805	182,284
Total	603,817	608,124
Total
Fair Value of Financial Assets and Liabilities
Long-term debt, including current maturities	518,301	534,068
Advances for construction	73,305	72,571
Total	$ 591,606	$ 606,639