Fair Value of Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Fair Value of Financial Assets and Liabilities
Risk premium (as a percent)	1.83%
Advances for construction	$ 194,046	$ 191,062
Cost
Fair Value of Financial Assets and Liabilities
Long-term debt, including current maturities, net	808,331	808,622
Advances for construction	194,046	191,062
Total	1,002,377	999,684
Total
Fair Value of Financial Assets and Liabilities
Long-term debt, including current maturities, net	989,749	873,454
Advances for construction	82,078	79,550
Total	1,071,827	953,004
Total | 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
Total | Level 2
Fair Value of Financial Assets and Liabilities
Long-term debt, including current maturities, net	989,749	873,454
Advances for construction	82,078	79,550
Total	1,071,827	953,004
Total | 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