Fair Value of Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Fair Value of Financial Assets and Liabilities
Risk premium (as a percent)	1.83%
Advances for construction	$ 196,853	$ 191,062
Cost
Fair Value of Financial Assets and Liabilities
Long-term debt, including current maturities, net	806,938	808,622
Advances for construction	196,853	191,062
Total	1,003,791	999,684
Fair Value
Fair Value of Financial Assets and Liabilities
Long-term debt, including current maturities, net	991,973	873,454
Advances for construction	82,150	79,550
Total	1,074,123	953,004
Fair Value | Level 1
Fair Value of Financial Assets and Liabilities
Long-term debt, including current maturities, net	0	0
Advances for construction	0	0
Total	0	0
Fair Value | Level 2
Fair Value of Financial Assets and Liabilities
Long-term debt, including current maturities, net	991,973	873,454
Advances for construction	82,150	79,550
Total	1,074,123	953,004
Fair Value | Level 3
Fair Value of Financial Assets and Liabilities
Long-term debt, including current maturities, net	0	0
Advances for construction	0	0
Total	$ 0	$ 0