Fair Value of Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Dec. 31, 2019
Fair Value of Financial Assets and Liabilities
Risk premium (as a percent)	1.83%
Advances for construction	$ 195,056	$ 191,062
Cost
Fair Value of Financial Assets and Liabilities
Long-term debt, including current maturities, net	807,129	808,622
Advances for construction	195,056	191,062
Total	1,002,185	999,684
Fair Value
Fair Value of Financial Assets and Liabilities
Long-term debt, including current maturities, net	979,858	873,454
Advances for construction	82,041	79,550
Total	1,061,899	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	979,858	873,454
Advances for construction	82,041	79,550
Total	1,061,899	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